feat(settings): option to keep opening MS Office files when another office app is enabled

[acbelle59]

Summary

Since #2301, when the onlyoffice (or officeonline) app is enabled, richdocuments stops handling Microsoft Office formats (docx, xlsx, pptx…) and only keeps OpenDocument formats, so that each app handles its own family of formats. That is a sensible default, but some admins want the opposite: keep opening every format with Collabora by default, and let users pick the other app from the file actions menu ("Open in ONLYOFFICE"), per document — see #3456 (and the earlier #928).

This adds an opt-in admin setting, msoffice_with_other_office_apps (default off, so nothing changes for existing installations). When enabled, hasOtherOOXMLApps() returns false and the MS Office mimetypes stay in the default list. It is exposed exactly like disable_certificate_verification: settings controller, admin initial state, capabilities, a checkbox in the admin settings (Advanced settings, under "Use OOXML by default"), and a paragraph in docs/app_settings.md.

How it was tested

Nextcloud 34.0.3, richdocuments 11.1.1, onlyoffice 10.1.2 (with the OnlyOffice connector's default formats released so that it does not claim the simple click):

• option enabled: a simple click opens Collabora; the file actions menu offers "Open in ONLYOFFICE"; the same document was edited with both editors (and from an iPhone with Collabora), all changes saved;
• option disabled: current behaviour unchanged — ONLYOFFICE takes the click, Collabora is not offered.

Resolves #3456.