Initial permissions remodel

[mathemancer]

Fixes #3625

• Moves all old DB-object models to a deprecated namespace
• Moves UIQuery model to Exploration
• Moves old Database model to Connection
• Adds new Server, Database, Role, UserDatabaseRoleMap models

It does not implement actually using the new models in any RPC functions. I was trying to keep the changes as small as possible.

Technical details

I tried to keep this PR minimal, but it's a major overhaul. The updated diagram for the relevant models is:

Note that Exploration isn't included in this diagram because it's not wired in yet. Also, I had to add some redundant foreign keys from the UserDatabaseRoleMap model:

• from database to Database
• from role to Role
• from server to Server.

These were necessitated by the way Django works. Basically, these were added since we want to be able to traverse those fields as related in python.

Also, the multicolumn foreign keys aren't visible in the service layer. They're just added as custom SQL migrations to enforce integrity.

Finally, all tests are currently passing, but no new tests are added. We'll add tests for actual functionality using these models as appropriate.

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the develop branch of the repository
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

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[Anish9901]

Looks great!