Feat: Use `Gmail` to send Email!! 📧 🤯

It recently occurred to me while reading the `swoosh` Adapters docs: https://hexdocs.pm/swoosh/Swoosh.Adapters.Gmail.html#content 👀
That I'm over-thinking/complicating things by using `AWS SES` for sending emails ... 💭

My _original_ reasoning for using `SES` is/was simple:
1. A dedicated purpose-built transactional email system that has bounce/failure detection.
2. Cheapest by _far_ and with a generous "free tier" means we can send **`62k emails / month`** for ***FREE*** before paying a penny ... see: https://aws.amazon.com/ses/pricing/
3. Fully customisable and usable from `AWS Lambda`, again "free tier".

These reasons are still technically valid. But they miss one important fact: **_setup_ complexity**!!
We ended up needing a whole separate repo for this: https://github.com/dwyl/aws-ses-lambda
Which we considered _reasonably_ well-documented/maintained ...
But doesn't have an `AWS CloudFormation` script to _automate_ the setup ...
so it's still a _manual_ process with several steps.

**What `if`** there was a **_dramatically_ simpler way** of doing this?
**What `if`** when setting up the `auth` app for the first time, the `admin` person
was able to sign-in with their `Google` Account e.g. `hello@app.io`
and then the `auth` app stored a token to send emails on behalf of the `admin` using their `Gmail`?!
This would solve _so_ many issues in one step!

# Simplified Setup

Consider the list of environment variables on the current `/init` screen: https://authdemo.fly.dev/init

1. The `ADMIN_EMAIL` would be populated by this step
so we would _never_ see the error described in #270
2. `EMAIL_APP_URL` would no longer be needed, i.e. no longer a _separate_ app! ref: https://github.com/dwyl/email/issues/77

# Enhanced Deliverability

Instead of having to waste time setting up all the enhanced verifications in a **3^rd** party email service,
Google handles the deliverability!!

## Replies!!

If people receiving an email from our `App` want to _reply_ they can, easily!
And that reply ends up in our collective inbox. Not in the `AWS SES` black hole.

# Limitations?

Google has a daily sending limit of **`500`** emails.

https://www.google.com/search?q=gmail+outbound+sending+limit+daily

This is totally fine during `MVP`, we _definitely_ won't have more than **`500`** signups per day.
For reference, this is **`15,000/month`** ... any bootstrapped startup would _dream_ of this!!
But what happens on a day that our `App` is mentioned on `HackerNews`, `ProductHunt`, `Reddit` or `YouTube`...?
Well, we build a "**waiting list**" feature https://github.com/dwyl/app/issues/304 that would store all the details and send the emails as soon as resources were available.