Upgrade from BS4 alpha 3 to 6

v4.0.0-alpha.6 is the latest version and they say the last alpha. They've refactored to using flexbox among other things so they'll be a little work to get the app into a working state.

• Preauth
• Nav bar
• Module nav
• Mobile nav
• Activity spinner
• Swap lesson mouseover dialogue to BS popover. Bad z-index in progress reports.
• Retro tool
• Award quals, hide select
• About logo
• Help section