Switch from Bootstrap 3 to Bootstrap 4

[TarEnethil]

Bootstrap 4 for flask is available (Flask-Bootstrap4). Archivar should switch to Bootstrap 4.

Things to do:

• [x] Replace Flask-Bootstrap with Flask-Bootstrap4
• [x] Check if static-CDN-Files need to be changed
• [x] Follow some transition guide i.e. https://getbootstrap.com/docs/4.0/migration/ to the get status quo up and running
• [x] Pay attention to and learn about things like flexbox
• [x] Look for alternatives for glyphicons (note: font-awesome is already used by SMDE)

[TarEnethil]

The switch to fontawesome has turned up some problems with SMDE, which are tracked in #28.

[TarEnethil]

The first migration is complete. All previous functionality is up and running again. There were many changes, so I expect many regressions as well, which will be fixed over time.

Major changes:

• Main Content Window is now slightly wider than before (col-8 in a container-fluid)
• Wiki is now wider as well (col-2 and col-10)
• replaced bootstrap-datetimepicker with temposdominus (works with Bootstrap 4)
• Flask-Bootstrap4 was installed, but the file imports are done manually (and locally tracked in static/css and static/js as well), because I wanted Bootstrap 4.4, which would have required to patch Flask-Bootstrap 4. In the long run, I will probably remove this package, but for now it has good utility (mainly the wtf.quick_form()).
• Added new functions to generate the in-page navigation in a consistent style
• Added new functions to generate (nav) buttons in a consistent style
• Changed Map Popup into a modal

This is it for now, but I will use more Bootstrap features as I overhaul modules anyway. The next big steps are probably sensible sidebars that integrate well into the design and work on mobile.