tooltip: Show only empty topic allowed tooltip on the topic text.

This commit shows "Only general chat allowed in this channel" tooltip
close to the topic text instead of in the center of topic input by updating
the topic input box "max-width" to "fit-content".

web/styles/compose.css

@@ -1198,6 +1198,7 @@ textarea.new_message_textarea {
     &.disabled {
         background-color: transparent;
         border-color: transparent;
+        max-width: fit-content;
     }
 }