Revert "style external links"

See merge request arsnova/frag.jetzt!506

This reverts commit c322a32c

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