Add necessary styles for views presented by Vue left menu

Add necessary styles for views presented by Vue left menu

Testing strategy

1. Add the feature toggle, node ./scripts/utils/feature-toggle.js --name vue-left-menu --description "Left menu using Vue (alpha)"
2. Enable the feature toggle
   • Enable: http://localhost:5000/api_web/features/vue-left-menu/1
   • Disable: http://localhost:5000/api_web/features/vue-left-menu/0

With the feature-toggle on, check out the following pages

• /home
• /home/explore
• /home/learn
• /community/home

UI to check (https://gitlab.com/gitlab-org/gitter/webapp/issues/2162):

• Search input in left menu
• Click the + menu bar item on the left menu and look for the blue popover item hover style
• Open up the create room and create community flows

With the feature-toggle off, general style inconsistencies on the same pages

Closes #2162 (closed)