Closes issue #30. Feature/issue 30/updating get data

The /libraries end point was updated to include:

num_users: the number of users that have permissions for a library.
- Only the admin and owner can see this information, otherwise it defaults to zero
num_documents: number of bibcodes in a library
date_created
date_last_modified
permission: finds the highest ranking permissions and returns that
Relevant tests have been included.
Migrations for the database changes included in this commit.
Added a few tests to ensure that permissions get deleted when a library is deleted.
Added a few tests to ensure the get libraries join is working as is expected.

adsabs / biblib-service