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 Even though this language matches the user story I think the feature should be more generalized to any user wanting a stable link to the library. 

```
In order to find primary sources in the holdings of a particular library
As a user
I want a stable link to search the holdings of a specific library.
```