GWTC models and API

• set up gwtc model and migration
• create urls paths for GET and POST operations
• set up views and serializers for api calls
• set up baseline permissions for external vs internal users
• set up catalog_managers group permissions for uploading and editing permissions
• create custom API calls, as needed

To be included in a future MR:

• create query language to recall and query gwtc superevents and events
• set up webpage views
• bug fixes and performance improvements