Make the GitLab issue template formatting configurable

### Problem Statement

The GitLab integration is difficult to use, because the issue description that Sentry populates into GitLab isn't customizable/template-able at all. Every issue we populate from Sentry to GitLab just defaults to this same hard-coded style of formatting:

This forces us to have to then _go into GitLab anyway_ and apply labels, assign people, etc. within GitLab. It completely defeats the point of having a Sentry integration with GitLab, since we have to go into GitLab to fix everything Sentry sends in anyway.

For example, we'd like to be able to always assign a set of labels and assign a person to things coming in from Sentry. GitLab makes this easy by putting `/label` slash commands in the issue description... but of course Sentry doesn't let us customize the issue description template so this isn't possible.

Can Sentry allow us to customize the template so we don't have to re-type all of the `/slash` commands on every issue or go into GitLab to do this for every issue?

### Solution Brainstorm

Add a setting, per project, that allows us to customize the issue template for the GitLab integration. A template could look like this:

```
## This is my custom template header

{{sentry_stacktrace}} <= Sentry would replace this with a stacktrace

{{sentry_url}} <= Sentry would replace this with a URL to the Sentry issue

Here is other stuff I can customize
```

### Product Area

Issues