The JavaScript side of the repository configuration form is very old,

and has existed as a bunch of flat functions that just used jQuery event

handling and global state to do anything.
For a long time now, we've wanted to revisit repository configuration,

but both the backend and frontend implementations have made this tricky.
To begin that process, this change ports the old JavaScript to a new

RB.RepositoryFormView class. This is a pure port of the old code,

quirks and all, with the idea being that we can then begin to more

easily clean it up piece-by-piece.
This does add some asserts to make sure we've correctly loaded all parts

of the DOM, converts gettext code to use our localization template

literals, and properly uses .prop() on checkboxes instead of .val().

Otherwise, nothing's really changed.
Future changes will start to move state into a model, and then begin

cleaning up this form to work like the rest of our codebase.
For now, there are no unit tests. This code isn't intended to work this

way long enough to be worth having tests. Those will come as part of

the modernization work.

Tested all aspects of the repository configuration form:

Configuring various tools for custom repositories
Configuring various hosting service repositories, with different tools

  and plans
Configuring bug trackers, with and without hosting services (and the

  "Use hosting service's bug tracker" checkbox)
Configuring ACLs
Re-editing hosting service credentials
Toggling SSH key display