[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

package.json

@@ -108,6 +108,7 @@
     "mocha": "^5.2.0",
     "mocha-circleci-reporter": "0.0.3",
     "npm-run-all": "^4.1.5",
+    "sass-loader": "^10.2.0",
     "shebang-loader": "^0.0.1",
     "spectron": "^14.0.0",
     "style-loader": "^0.23.1",
@@ -136,6 +137,7 @@
     "react-bootstrap": "^1.6.1",
     "react-dom": "^16.14.0",
     "react-transition-group": "^2.5.0",
+    "sass": "^1.35.1",
     "semver": "^5.5.0",
     "underscore": "^1.9.1",
     "valid-url": "^1.0.9",