Add .ics to allowed file extensions by default, and add a calendar file kind

[MoritzLost]

iCalendar (.ics) files are the common exchange format for calendars / events. I just noticed that they are not allowed by default in the allowedFileKinds general config, meaning you have to allow them manually via extraAllowedFileKinds.

iCalendar files are plaintext and aren't used for any known exploits as far as I can tell, and they're pretty common, so there's no reason not to allow them by default.

Suggested improvement

• Allow ics in allowedFileKinds by default.
• Add a file kind calendar with that extension to the list of default file kinds.