Integrate Plugin Check into plugin submissions

[dd32]

It's been long requested to automatically run plugin-check over plugin submissions automatically.

[dd32]

In 14063:

Plugin Directory: Add code to run newly submitted plugins through plugin-check.

At present this doesn't show anything to the submitter, pending confirmation of implementations.

See #7778.

[dd32]

In 14064:

Plugin Directory: Plugin Check: When the plugin is not available, return the expected array struct.

See #7778.

[dd32]

In 14066:

Plugin Directory: Plugin Check: Make the Slack logs easier to glance through.

See #7778.

[dd32]

In 14069:

Plugin Directory: Plugin Check: Specifically request only the plugin_repo checks and ignore some warnings.

Props rabmalin.
See #7778.