Clarify app manager method names

[come-nc]

• Resolves: #

Summary

Renames isInstalled -> isEnabledForAnyone, and getInstalledApps -> getEnabledApps.

There is a weird situation for apps which are enabled only for some groups of users:

• They are in the return of getInstalledApps/getEnabledApps
• They return true for isInstalled/isEnabledForAnyone (which is why I named it that way)
  For isEnabled*, one can use isEnabledForUser($app, null), to check if the app is enabled for everyone.
  But for getEnabled*, there is no equivalent, as getEnabledForUser parameter is mandatory.
  Because of that, in NavigationManager, isEnabledForUser is called on each app returned by getInstalledApps.

We could consider that when there is no user in session we should always exclude apps enabled only for some groups, and I think for public pages it makes sense (does it?).
But the issue is for occ cli tool, there there is no user in session but we do want to consider app enabled for some groups only as enabled.

So basically for each getInstalledApps call, we should ask ourselves whether the list should include partially-enabled apps or not, and replace with the appropriate method call.
We may want to rename getEnabledApps with a more precise name, but getEnableAppsForAnyone is ugly, no?
We may want to add a way to get apps enabled for everyone, either as a new method or by making the parameter nullable in getEnabledAppsForUser.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[come-nc]

@nickvergessen Thoughts on the naming and logic?

[provokateurin]

LGTM, just some of the commits should be squashed

[come-nc]

rebased and squashed some commits