Feature/stable repo links

Hi @barnabyalter 

All tests are passing here now, but deploying this will require incorporating the companion pull request in the findingaids_ead repo and deleteing/reloading tamwag/tamiment, archives/universityarchives, bhs/brooklynhistory, and nyhs/nyhistory.

I had one question, which I'm including as an inline comment in the code below.

Let me know if you have questions or comments

cc @NYULibraries/finding-aids 
@chrpr You shouldn't need this at all. The routes will automatically select the correct respository value