Make it easy for plugin review team to add plugins to the Block Directory

[tellyworth]

It should be easy for the plugin review team to see the results of the block plugin validator for a plugin, and add or remove plugins from the Block Directory.

[tellyworth]

Add a Review Tools block to the validator page, with an Add/Remove button

[tellyworth]

Improved patch

[tellyworth]

In 10022:

Plugin directory: add initial tool for plugin reviewers to add blocks to directory.

Props tellyworth, dd32, dufresnesteven.
See #5306.

[ryelle]

reviewer-ui.diff​ removes the fieldset wrapper to match the hierarchy on the rest of the page (since it doesn't add anything semantically, and might even be confusing for screen reader users). I also tweaked it to not allow adding to the block directory if there are any error-level results, so it could be a step to letting devs manage this themselves.

[ryelle]

In 10036:

Plugin Directory: UI Improvements for reviewer section

See #5306