FIX: Hide full screen toggle button when textarea is disabled (PR #14638)

When the composer is disabled (happens on sites where a category is required and at least one category has a topic template), the full screen toggle is not useful because it will show only a disabled textarea. It’s better to hide the full screen toggle button in this case.

A second, cosmetic change: the toolbar is shown at half opacity when the textarea is disabled.

Before:

After:

GitHub