Make open_athens_subscope an array in the insitution model

Issue

An institution needs to be able to configure multiple openathens subscopes, e.g., a public library that has multiple branches

Test scenario

Flint River has two branches

So we need to add all the subscopes for Flint River like this

Then we can

• create personal accounts in openathens admin for Flint River, Fayette County, and Peachtree City.
• go to myathens in incognito mode and log in as one of these accounts.
• go to galileo (staging or prod depending where we're testing)
• in the wayfinder, select any openathens enabled institution (Flint River isn't enabled yet)

What should happen is that no matter which account you logged in as (Flint River, Fayette, or Peachtree), galileo should log you in as Flint River.