Native support for OP badge (original poster) in forums

[tobifjellner]

For moderators it is often important to quickly see if a reply is from the original poster / thread starter.
For this purpose, we're using a nice browser add-on by @clorith ​https://github.com/wporg-support/topic-op
But I think it would be better to have this functionality built-in directly on WordPress.org. This would make it possible to see directly in the list of pending/spammed/ posts if a thread reply was made by the topic starter, which would allow the moderators to make quicker and better decisions (by easier noticing "me too" or "thread hi-jacking" attempts.

These badges could be visible to everyone, or just to moderators/admins, let's see what others say.

[dd32]

In 10666:

Support: Add a "Thread Starter" (OP) badge to the thread starters replies.

This uses the same styling as moderators/plugin & theme authors/support reps, but with a green colour.

The design is experimental, and might need adjustment.

See #5611.

[dd32]

The above implements this as a "Thread Starter" label with a green colour.

I've copied the styling from the other badges, but I feel it might be too noisy. I'm tempted to leave the top badge, but remove the side border, for example, the following upload includes the current styling (example) and removing the border (example2)

[dd32]

In 10672:

Support: Remove the solid border from the OP replies.

See r10666.
See #5611.

[dd32]

In 10709:

Support: User Badges: Remove the final parts of the solid border colour for the thread starter.

Follow up to [10672].
Fixes #5611.