MM-15562 Add a way to unregister post type components

[hmhealey]

This is needed for the NPS plugin because it stops using its custom post component at small screen sizes when there's not enough room to display it properly.

The REMOVED_PLUGIN_POST_COMPONENT action is already handled by the correct reducer, but there was nothing that actually emitted it.

Ticket Link

https://mattermost.atlassian.net/browse/MM-15562

[crspeller]

So it detects the screen size change and unregisters it's component?

[hanzei]

I'm still not knowable enough to review this. Please request a review from someone else please.

[hmhealey]

Oh right, I forgot your focus was on the server-side, hanzei. I'll re-assign it

[hmhealey]

@crspeller That's correct. I would've just had it implement it's own dropdown, but the fallback uses the post action dropdown which is pretty rich in features, and I didn't want to copy that over. This function was already used by the NPS plugin, but I forgot to actually submit this PR, so that behaviour is currently broken.
https://github.com/mattermost/mattermost-plugin-nps/blob/master/webapp/src/index.js#L21