RFC: GithubRepo is modeled incorrectly

# Problem

#1080 added a unique index to the `github_repos` schema.

#1081 also changes behavior when syncing github repos to match by `github_id`

The model for a `GithubRepo`, however, looks like this (important bits only):

```
schema "github_repos" do
field :github_id, :integer

belongs_to :github_app_installation, CodeCorps.GithubAppInstallation
end
```

The problem here is, in a real world, a github repository is not uniquely identified by an installation, but by it's `github_id`, as we are aiming to change.

In our specific case, a `belongs_to :github_app_installation` simply tells us which installation provided us with the information about the repository.

I guess our initial goal was to model it so each installation stores it's own copy, even though, on github, it's the same record. With the addition of other records, this no longer works optimally.

However, with the way it all works, since in our model, a repo can only belong to a single installation, we will encounter A LOT of difficulties as we start dealing with multiple installations.

## Proposed general solution

- remove the "belongs_to" relationship
- add a "link" table and change it to a many-to-many relationship
- not sure about naming `GithubAppInstallationGithubRepo`, `GithubInstallationRepo`? `GithubAppInstallationRepo`, `InstallationGithubRepo`? `GithubAppInstallationRepo` sounds best to me.

## Specific changes needed

In `CodeCorps.GitHub.Event.Installation.Repos`

- the `:delete_repos`, `:sync_repos` steps are replaced with
- `:ensure_repos` - find or create al repositories
- `:disassociate_repos` - delete `GithubAppInstallationRepo` records no longer in the installation
- `:associate_repos` - create `GithubAppInstallationRepo` records in the installation