Resolve "Questions should be filtered by creation period"

Closes #543

See merge request arsnova/frag.jetzt!505

For keeping the active filter is now saved in a variable.

A new button is now displayed in comment list view for creators and participants. A click on the button will open
a menu with which the user can select a time period for the displayed comments. Sorting and filtering actions will
applied to the selected period. Displaying all existing comments is the default setting.

Merge branch '566-fix-session-list-title-containing-number-of-messages-from-last-visited-session' into 'master'

fix session title bug

Closes #566

See merge request arsnova/frag.jetzt!504

add commentCounter to title

Closes #565

See merge request arsnova/frag.jetzt!503

enable local test coverage analysis

Closes #563

See merge request arsnova/frag.jetzt!502

Resolve "Enable static code analysis with Sonarqube for frag.jetzt frontend"

Closes #558

See merge request arsnova/frag.jetzt!501

Resolve "when motd is read, add it to old messages"

Closes #562

See merge request arsnova/frag.jetzt!500

sort motd anti-chronologically

Closes #557

See merge request arsnova/frag.jetzt!499