aggregate of multiple small ui cleanups and polishes

also includes

• bumping bootstrap to 4.5.3, jquery to 1.5.1
• add popper.js 1.16.1 for nice dropdowns
• cleanup button tohuwabohu over at the session overview
• add session settings page to be able to change the invite text, meeting link and session start
• add option to add additional election managers to a session in the session settings overview
• also add a Makefile to easily run linters and tests and update the ci to reflext that