Fix some of the Javascript linting issues, as well as bugs and unneeded divergences with upstream (#2208)

* Run eslint --fix

* Fix linting issues in video player and reduce divergences with upstream

This includes a behavior change of not auto-looping videos anymore. I don't
remember loops being ever intended, and they have been removed from upstream
a while ago, but we somehow missed the change.

* Fix lint issues in `app/javascript/flavours/glitch/selectors/index.js`

Those were basically caused by dead code that isn't present upstream, so
that brings us closer to upstream as well.

* Fix linting issue and bug in streaming/index.js

* Fix linting issues in config/webpack/shared.js

* Fix unused import in flavours/glitch/features/ui/index.js

* Fix linting issues and reduce divergences from upstream in flavours/glitch/features/ui/components/video_modal.jsx

* Fix linting issues in flavours/glitch/reducers

* Fix linting issues in glitch-soc onboarding modal

* Fix linting issues in flavours/glitch/features/ui/components/navigation_panel.jsx

* Remove dead code for unused local setting navbar_under

* Fix various linting issues

* Fix linting issues in flavours/glitch/components/scrollable_list.jsx and reduce divergences with upstream

app/javascript/flavours/glitch/reducers/compose.js

@@ -446,6 +446,7 @@ export default function compose(state = initialState, action) {
     });
   case COMPOSE_REPLY_CANCEL:
     state = state.setIn(['advanced_options', 'threaded_mode'], false);
+    // eslint-disable-next-line no-fallthrough -- fall-through to `COMPOSE_RESET` is intended
   case COMPOSE_RESET:
     return state.withMutations(map => {
       map.set('in_reply_to', null);

app/javascript/flavours/glitch/reducers/local_settings.js

@@ -8,7 +8,6 @@ import { LOCAL_SETTING_CHANGE, LOCAL_SETTING_DELETE } from 'flavours/glitch/acti
 const initialState = ImmutableMap({
   layout    : 'auto',
   stretch   : true,
-  navbar_under : false,
   side_arm  : 'none',
   side_arm_reply_mode : 'keep',
   show_reply_count : false,