Either don't show "create topic" or show an error for non-logged-in user

[sterndata]

See ​https://wordpress.org/support/topic/two-problems-in-wordpress-org-forum

" when “non-logged-in” user click “create topic“, nothing happens.

​https://i.imgur.com/Gw0A7PU.png
At least, there should be some message, that he is not logged in or etc…"

[dd32]

In 7875:

Support Forums: Replace the forum Create Topic button with a 'Login to Create a Topic' button when logged out.

Fixes #3934.

[SergeyBiryukov]

In 8693:

Support Forums: Replace "login" (used as a verb) in a link to new topic form with "log in".

See #3934.