#477 and #178 Misc bugfixes and small edits to the initial Version/Library import and the monthly refresh

[williln]

Closes #477. Part of #178.

• Skip versions prior to 1.0 -- those don't include the libraries in .gitmodules and there is not currently another way to get that data
• Make library and version on LibraryVersion required
• When adding library-versions, if the library doesn't exist, refactored to add the library instead of skipping it
• Some minor bugfixes
• Some minor refactoring to make running the commands and understanding the code easier, including making update_library() more generic and only updating the db, and moving the other update methods to within the loop in update_libraries(). Also added calls to get PR and issue data that were missing before (methods were there, just not called), but commented it out. It's data that takes a long time to pull, so we may want to either break up the process into celery tasks, or add logic to handle rate limiting, or both. CC @4down @frankwiles since this PR/commit data appears on the new homepage I think?
• Refactor to get all commit data on the initial load (./manage.py update_libraries), but only the most recent month in the monthly refresh task