Persistent Chat: fix de-minimize misclick

Summary:
When the persistent chat is minimized to the bottom of your window, as mentioned in T15626,
the chat bar still has an invisible square area that does nothing if you click it by mistake:

{F2184778}

That area was reserved for the Settings Icon, that appears only if you de-minimize the chat.

The Settings Icon was toggled incorrectly from CSS. It was just hiding the Font Awesome icon,
(.fa-gear), instead of hiding its clickable container.

After this change, chat can be de-minimized even if you click in that specific evil spot.

Original credit to @roberto.urbani for the original troubleshooting and original patch.

Follow-up from: D25428

Closes T15626

Test Plan:
Show the Chat, then:

- when the chat is de-collapsed:
- you see the Settings icon (as before)
- you can click on the Settings Icon (as before)
- it shows the Settings menu (as before)
- when the chat is collapsed:
- you can click in whatever point, and it de-collapse (better than before)

Additionally, use the HTML inspector in your browser (`F12` from Firefox),
to double-check that now that `<li>` has display: none when the chat is collapsed.

Reviewers: O1 Blessed Committers, aklapper

Reviewed By: O1 Blessed Committers, aklapper

Subscribers: aklapper, tobiaswiese, Matthew, Cigaryno, roberto.urbani

Maniphest Tasks: T15626

Differential Revision: https://we.phorge.it/D25628