Translation interface

Add basic translation form, with ability to add new pages as well as edit existing ones. Also a table of page vs translation statuses.

New routes: /translations/{toolName} and /translate/{toolName}/{path}.

This isn't a complete feature, but I'm not sure how far to progress with it because I think there a whole bunch of things that need more discussion. Hopefully that discussion will be easier with this somewhat-working prototype in front of us.

Bug: #308997