FontAwesome 5

Closes #982 (closed)

Replaces all icons with FA5 counterparts. Removes Fontello

Some icons changed to a (slightly) different icon:

• All spinners were changed to circle-notch since FA doesn't have any spinners beyond 7-dots and circle-notch ones.
• bell-ringing is instead a frankenstein of bell and two rss icons to simulate ringing, since bell-on is a FA PRO icon :/
• Video icon (in hidden attachment placeholder) uses generic camera
• Poll icon uses horizontal poll icon, i.e. similar to graph but horizontal and inside a square.
• Smile icon uses beam version, i.e. it does the ^_^ expression. Because people complained that plain :) "stares at them"
• Smile icon in post form changed to filled variant for better harmony with other icons
• globe is changed to a generic "basketball"
• lock now has shackle turned over for better accessibility
• ellipsis in usercard is vertical instead of horizontal
• version tab in settings is now information without circle.
• notifications tab in settings is non-ringing bell
• maybe more i can't remember.

Some small bugfixes:

• Icons in navbar have been readjusted
• Icons in user-card have been readjusted and given MUCH bigger hitbox + better hover indication.
• Some components like RetweetButton have been refactored.
• Some components's CSS was changed to fit better with our guidelines.