Fix map legend translation

This fixes a number of problems with translation and the legend:
- Django was returning strings in English regardless of the translation cookie; this was because the text was being pulled from a CharField, which can't be translated, rather than the GTFSRouteTypes.CHOICES variable, which can.
- The legend was not being re-fetched by Angular after a translation change due to caching.
- The Angular interface was not respecting the language cookie and would always load in English on the first page load.

There are a number of asynchronous things that happen when translations are changed, and on Friday I was having some trouble with apparent race conditions when updating the legend. However, I haven't been able to replicate any of them this morning, so we can deal with them according to their severity if they come up again.