Add available GithubAppInstallations to a user

# Problem

We need to find a way to add available `GithubAppInstallations` for a `User` that the user has access to for the GitHub App.

We'll have to refresh this data by hitting [GitHub's API endpoint](https://developer.github.com/v3/apps/#list-installations-for-user) for listing a user's installations.

We may want an endpoint for this at our own `/user/github-app-installations`. We could also consider having this simply be available to a `User` on the `UserView`, but this may confuse things with an existing `GithubAppInstallations` relationship. We could perhaps rename the key to something like `available-github-app-installations` but keep the type `github-app-installation`.

Alternatively, we could also rename the `user` relationship in `GithubAppInstallation` to `sender`, which is more in-line with GitHub's own API and can reduce some confusion there.