BuddyPress: Banned group member can still post if form is visible

[r-a-y]

Originally posted on BuddyPress Trac:
​https://buddypress.trac.wordpress.org/ticket/6835

This is a follow-up to #2095. #2095 hides the post form for a banned group member, but if the banned group member presses the "Back" button in their browser prior to the ban, they can still view the post form and is able to create additional replies.

We also need to check the permissions before the post is actually submitted.

Attached is a patch that does this.

[thebrandonallen]

Updated patch uses meta caps to block a banned user with access to a topic/reply form from posting.

@r-a-y Can you confirm that this still works as intended?

[r-a-y]

Nice approach, @thebrandonallen.

However 2.diff didn't work for me, but your patch gave me an idea! 03.patch should work for our needs.

[thebrandonallen]

@r-a-y Thanks for getting back. If it works for you, it works for me :)

[johnjamesjacoby]

In 6229:

BuddyPress: Prevent reading/publishing if group member is banned.

Fixes #2905. Props thebrandonallen, r-a-y.