Offer more universal support on top of GitHub

A few other things can be made configurable to support Launchpad, GitLab, and other Git hosting providers.

### Reporting issues

The feedback button is still hard-coded to support GitHub only. We should make it possible to use it to file bugs elsewhere.

- rename it to `tracker_issues` or some such
- rename associated stuff in JS (`github-issue-link` etc.)
- add option for setting the bug-tracker URL (or offer options for Launchpad and GitLab); for example, for Launchpad, it would be something like `https://bugs.launchpad.net//+filebug?field.title=value`

### Repo URL

We still use a `github_url` parameter.

- this should be changed to `repo_url` similarly to `repo_folder` etc.
- code that consumes it should be updated