fix(core): make the app store link in the app menu configurable

[pringelmann]

• Resolves: [Bug]: new app menu for Nextcloud 34: don't show "app store"-button for normal users #60495

Summary

Non-admins get an "App store" tile linking to apps.nextcloud.com with no way to turn it off.
This introduces a new config key appstore_link_shown, with preset defaults:

Preset	Link
none, private, family, club	shown
shared, school, university, small, medium, large	hidden

An explicitly set value always wins. Otherwise the link is hidden when appstoreenabled is off or a subscription is available, else the preset default applies. No preset means the link stays, so upgrades don't change.

This does also broaden appstoreenabled. It used to only gate installing from the store, now it hides the link too.

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)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI (tests, reviewed)

[jancborchardt]

Nice refinement, as discussed :)

[pringelmann]

/backport to stable35

[pringelmann]

/backport to stable34