Add support for the `editor` role

[nkaretnikov]

Fixes #675.

Description

This pull request:

• adds the alias editor for developer to role mappings, which is supported by the HTTP API and the DB
• adds tests
• updates documentation.

The auth flow is as follows:

• an HTTP API is called to set the role (can be either editor or developer)
• the role is written to the DB (always written as developer)
• when some page is accessed, the role is checked on auth by matching the DB value against role_mappings
  • developer from the DB is matched against editor or developer in role_mappings (in this order).

All user-visible parameters (the HTTP API and the role_mappings setting) allow for both editor and developer. The name editor is preferred, which is reflected in the documentation. The name developer is supported for compatibility reasons.

How I tested this locally:

• [x] to make sure that role_mappings are actually used by the test, I commented out editor in the default role_mappings
  • the test errored out as expected
• [x] to make sure that developer is still supported, I replaced editor with developer in the default role_mappings
  • the test succeeded as expected
• [x] to make sure that permissions are checked, I replaced the permissions of editor in the default role_mappings
  • the test errored out as expected

After making each of these changes in the application, I ran

PYTEST_ADDOPTS="-rpfxs -k test_end_to_end_auth_flow" pytest tests

and looked at the test output.

Pull request checklist

• [x] Did you test this change locally?
• [x] Did you update the documentation (if required)?
• [x] Did you add/update relevant tests for this change (if required)?

Additional information

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[nkaretnikov]

@dcmcand PTAL. Note: I want to keep the diff minimal to make it easier to review, I'll rebase after this is approved.