Connections API

[mathemancer]

Fixes #3291

This PR is an initial iteration towards replacing the "Database" concept in our codebase with a "DB Connection" (usually just "Connection") concept.

Technical details

The following endpoints are changed:

• /api/ui/v0/databases/* -> /api/ui/v0/connections/*
• /api/db/v0/databases/* -> /api/db/v0/connections/*

Note that as indicated by the wildcard, all endpoints under these namespaces are affected.

Changes to keys in JSON representations of connections (orig databases):

• name -> nickname
• db_name -> database
• deleted -> (removed)
• editable -> (removed)
• error -> (removed)

Changes to common data JSON keys:

• .current_db -> .current_db_connection
• .databases -> .connections
• .databases.[i].name -> .connections[0].nickname`
• .databases.[i].db_name -> .connections[0].database`
• .internal_database -> .internal_db_connection
• .internal_database.db_name -> .internal_db_connection.database

Note that in order to keep this PR small, the Schema model refers to its database rather than connection in the API and common data.

Behavior changes:

When creating or updating a connection (formerly database) via POST or PATCH, we no longer test the actual connection or install Mathesar on the DB. That will be handled by a subsequent PR on the RPC endpoint.

Regarding MATHESAR_DATABASES:

I left that variable for now, since removing it would require rewriting major pieces of our back end test suite. However, I don't think it will affect much. I think we should remove that variable from our docs and examples, except for the dev setup, where having a preinstalled DB automatically created is convenient.

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• 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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[mathemancer]

@Anish9901 Please review, but don't merge until @seancolsen makes the required front end changes.

[Anish9901]

Looks great @mathemancer! I've addressed a small bug please take a look.

[Anish9901]

There is a minor bug here, the uri formed will always be namespaced under ui/ as the basename is same for both the db & ui route. Not sure how significant it is for the frontend though.

[mathemancer]

Hmmm. I'll experiment with it a bit.

[mathemancer]

@Anish9901 I poked around and found that the behavior's the same as before. I agree that this is a bit of a bug, but it's apparently harmless. Given that we're going to be completely overhauling the API structure in the near future, I want to avoid fixing it for now.

[pavish]

I'm taking over the frontend work from @seancolsen for changes needed in this PR.

[pavish]

@mathemancer @Anish9901 I've fixed the breaking changes on the frontend, except the previous 'Database Connections' settings page which will be rewritten as per the new spec. This PR can be merged when ready.