Reorg namespacing

[mathemancer]

Fixes #3797

This PR reorganizes and cleans up some of the RPC function code and naming.

Technical details

The PR makes the renaming changes in the issue, and also:

• removes an unneeded parameter from list_db_priv (no effect on API)
• Modifies the response a bit from databases.get (previously database_privileges.get_self to conform with what one would expect from such a function. The information is a superset of what was previously returned.

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• I added tests for the changes I made (if applicable).
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  visible errors.

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[mathemancer]

@Anish9901 Please go through this one a bit carefully, since there were so many renamings.

@pavish This is ready to wire up with the front end.

[mathemancer]

@Anish9901 This is ready for backend review, but I'm marking it as 'draft' since @pavish plans to make the front end changes in this same PR.

[pavish]

I've wired up the frontend with the changes.

[Anish9901]

LGTM!