[MM-36421] Replace server tabs with dropdown menu (#1647)

* WIP

* WIP

* PoC for dropdown - logic stuff

* Most of the logic for the dropdown

* Working dropdown menu to pick servers

* Mentions, unreads and expired working. Many styles are working.

* Some more styles

* Dark mode and other stuff

* Some cleanup

* Generate build

* PR feedback

* More PR feedback

.circleci/config.yml

@@ -446,7 +446,7 @@ workflows:
             branches:
               only:
                 - /^release-\d+(\.\d+){1,2}(-rc.*)?/
-                - react_bootstrap_upgrade
+                - pull/1647
 
       - store_artifacts:
           # for master/PR builds