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

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• bb91b517 - dont install python3.7 in ci anymore

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• cd0dcafe - moved template variable description from view to form in add election (like we...

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mentioned in commit f1c6533a

merged