reduce multiselect max-width from 400 to 200px to not take too much space

Test this in the user management with some users who are in lots of groups. Currently this leads to a very wide table, which can result in a horizontal scrollbar and not being able to delete a specific user because the name is on the far left (out of viewport) when the delete icon is visible on the right.

This is a quick fix for that, please review @owncloud/designers @msrex 

(Note to self: We should also backport this to stable6.)