IGAPP-1152: Export events api client

[Cmd190]

This pull request also contains IGAPP-1129.
How to test it: Go to Events => Click on Item => click on export button

Import Google Kalendar: Settings -> Import & Export
For other applications: Just open ics file

[f1sh1918]

Really nice work. Tested on apple calendar and google calendar.
I have commented some minor adjustments

[steffenkleinle]

Tested on web, works fine. In the designs the background of the button is grey instead of yellow, why did you choose yellow?

[f1sh1918]

Tested on web, works fine. In the designs the background of the button is grey instead of yellow, why did you choose yellow?

Just added comment to design, since we should stick to our standard <TextButton> if possible and they are yellow. I would at least suggest to use same style everywhere

[LeandraH]

Looks good, tested in Firefox. Have to concur with Steffen though, in the design the button is grey and wider. Nvm, just saw your previous comment about it.

[ztefanie]

Looks good, testes on Chrome

only one small thing:
Line length is too long for ical format (but it is only warning not error level)
https://icalendar.org/iCalendar-RFC-5545/3-1-content-lines.html

[f1sh1918]

hm do you think we should force adding new lines? I imported in several calendars and couldn't find any issue. It will look very odd to randomly add news lines every ~20 chars @SteffiStuffel