Plugin Directory: Prevent BANNED users from being added to plugins

[Ipstenu]

Currently you can add a banned user as a committer to a plugin.

We should block this, as if someone's been banned, we don't want them messing with plugins for a reason, most likely security related.

[obenland]

To fix this, we could probably reuse the blocked user check from the author card meta box, in API\Routes\Plugin_Committers::add_committer() and Admin\Metabox\Committers::add_committer().

[obenland]

In 7415:

Plugins: Prevent BANNED users from getting plugin commit

Props dryanpress.
Fixes #3700.

[obenland]

In 7420:

Plugins: Fix typo.

Props presskopp.
See #3700.