Allow configuring separate public and internal collabora urls

[juliushaertl]

While we have a hidden app config value for the public wopi url, it would be more straight forward to have this configurable in the UI. This would also allow us to remove some flaky logic to dynamically set the public url.

• Collabora URL (used by Nextcloud server)
• Public Collabora URL (used by the users browser)
  • Fallback to default (internal) Collabora URL if not set (for most cases)
• Internal Nextcloud URL (can be used to set a different callback url for Collabora to send WOPI requests to Nextcloud)
  • Fallback to current request server URL (for most cases)

This would allow direct communication between Nextcloud/Collabora which can be beneficial to avoid always going through reverse proxies.