Build and publish Debian packages for Red

Hello, after a discussion in AC, I started working on a Debian package (`.deb`) for Red-DiscordBot, with all the automated stuff.

Doing this will make the install process as easy as possible on all Debian-based Linux distros, such as Linux Mint or Ubuntu. Installing the bot would only require a single command:
```
sudo apt install red-discordbot
```
Python, git, dependencies, venvs, everything is included and the next command would simply be `redbot-setup`. As easy as this.

## Current status

For now I'm messing inside an ubuntu-server virtualbox, see if I can successfully build a debian package and make it work. Then I will work on automated builds and publications.

I'm using `dh-virtualenv` to automatically build the Python package, and it works pretty well! This is the test and crash part, I'm far from opening a PR, which means it's the time to discuss about your concerns and suggestions about this.

With the setup below, I got a *mostly working* Debian package. The binary files are located over `/opt/venvs/`, I need to figure out how to add this to the path, or copy the files to a known directory included in path.

Once this works, I'm gonna have to write tests and automated builds with Github Actions, plus the publication to Debian/Ubuntu archives.

## How to test

If you want to try what I've done so far, here are the steps (for Ubuntu 20.04):

1. Install the deps
```
sudo apt-get install build-essential debhelper devscripts equivs python3-all python3-setuptools python3.8-dev python3-pip dh-python git openjdk-11-jre-headless
```

2. Install `dh-virtualenv`. There's no source for Ubuntu 20.04 yet, so you must [build from source](https://dh-virtualenv.readthedocs.io/en/1.2.1/tutorial.html#step-1-install-dh-virtualenv).

3. Clone the repo, gud fresh install, with the name of the release
```
git clone https://github.com/Cog-Creators/Red-DiscordBot red-discordbot_3.4.0
```

4. Create a `debian` folder inside the cloned repo and insert the following files (subject to change. a lot):

* `control`
```
Source: red-discordbot
Section: python
Priority: extra
Maintainer: "Cog-Creators"
Standards-Version: 3.4.0
Build-Depends: debhelper (>= 11~), dh-virtualenv (>= 0.8), python3-all, python3-setuptools, python3.8-dev, python3-pip, dh-python, git, openjdk-11-jre-headless

Package: red-discordbot
Architecture: any
Depends: ${python3:Depends}, ${misc:Depends}
X-Python3-Version: >= 3.8.1
Homepage: https://github.com/Cog-Creators/Red-DiscordBot
Description: A highly customisable Discord bot
```

* `rules`
```
#! /usr/bin/make -f

%:
dh $@ --with python-virtualenv
```

* `changelog`
```
red-discordbot (3.4.0) UNRELEASED; urgency=medium

* Initial release.

-- El Laggron Mon, 17 Aug 2020 22:49:36 +0000
```

* `compat` (it's important I swear, don't omit)
```
9
```

Check that you have 4 files with `ls debian`.

5. Build at the root of the repo
```
( deactivate ; dpkg-buildpackage -us -uc -b )
```

If the build is successful, you will have plenty of files in the parent (`..`) directory. Look for the `.deb` and try to run `apt install ./file.deb`.

If the install is successful, you should be able to see the result over `/opt/venvs/red-discordbot/`, that's where the binary files are.

## Basic issues

- [ ] The venv isn't writable. Needs to modify the permissions of the dir on install.
- [ ] The bin files aren't in path. Need to either make the install process add `/opt/venvs/red-discordbot/bin` in PATH, or make a syslink inside `/usr/bin`, which is [the option recommended by `dh-python`](https://dh-virtualenv.readthedocs.io/en/1.2.1/howtos.html#making-executables-available).

## Issues that needs discussion

### The installed venv is system-wide, which is a radical change from a normal redbot install

A problem raised by an install with `apt` is that the installation location (and the env) becomes system-wide, instead of user, or venv wide. This means that, if you create multiple instances, even with different users, __they will share the same environment__.

There is still a venv, it will not conflict with the other packages that can be installed. It's just an unique venv that, for now, can't be duplicated.

This problem isn't actually new, but apt makes it harder to fix, because it's theoretically not possible to create multiple venvs with the same apt install. If people want a different venv, they would have to roll back to the initial installation method.

What could we do to fix this? Not changing code on debian package's end, but directly in core Red, to allow for example one venv per instance?