Add Route movement and related methods

[meshy]

Deferred from #75:

• [x] Rebase once #75 is merged.
• [x] Add Route.objects.move_branch(old_url, new_url).
• [x] Add Route().get_ancestors()
• [x] Add Route().swap_with(other_route, move_children)
• [x] Add Route().move_to(new_url, move_children)
• [x] Decide if these method ought to be in atomic transactions or not. Conclusion: not at the moment.
• [x] Determine if validation is worthwhile on the above methods. Conclusion: yes, but optionally disabled with flag. Defer to another PR.
• [x] Raise error from get_ancestors when called on object that hasn't yet been saved to the database (has no pk).
• [x] Mark user-facing strings as translatable.
• [x] Consider re-writing long queries to clarify their single-query nature (remove line-smearing variables).

Trashed ideas:

- [ ] Add Route.objects.swap_urls(first, second) method. - [ ] Add Route.objects.swap_branches(first, second) method. - [ ] Add Route().get_parent()

[meshy]

@KevinEtienne @Ian-Foote thanks for the feedback guys, but it's probably not worth reviewing this yet, as I'm still working on it, and #75 needs to be merged in first.

[meshy]

I very much appreciate the feedback, BTW :)

[meshy]

Oh, just realised what was going on there... I was asking questions of myself to answer later! Should have known better! Glad to have your answers, and now feel bad for stopping you! :P

[meshy]

Ok, I think I've addressed everything. I've refactored pretty heavily to reduce churn in the history.

@Ian-Foote might I ask for another look and (if you're happy) a 👍?

[meshy]

\o/ Thanks! :)