Apps page pulls in all groups at once, can cause long loading time

[blizzz]

By https://github.com/owncloud/core/blob/master/settings/apps.php#L32 all groups are loaded. On bigger LDAP setups (my test setup here > 7k groups) this will lead to long loading times.

To reproduce:

1. create a huge amount of groups (preferably with LDAP)
2. open apps page

cc @icewind1991

[MTRichards]

#9014 #9201 Related

[PVince81]

We should probably only load the groups when needed, which is in this specific situation:

1. App is enabled (or has just been enabled)
2. "Enable for groups" checkbox is checked

Only then load the groups, maybe show a spinner instead of the select box, then show the select box.
That's still not lazy loading like for #8507 but still a better experience in general for the apps page.

[PVince81]

Another idea is to just use select2 with ajax like it was done here #10558

[PVince81]

Fixed by #10968