Add CSR for photoframe app

[jeppester]

Link to app:
https://github.com/jeppester/nextcloud-photo-frame

Description (also in README):
Photo Frame lets you generate photo frame URL's for your albums which can then be viewed on any screen with a browser.

The possiblities are endless:

• Use kiosk browsers (like Fully Kiosk Browser) to turn any android device into a photo frame
• Use collaborative photo albums to co-create and share photo frame content across households

Important
The app is functional in the way that I'm using it in my own instance, but it's not 100% ready to be published yet (for instance it lacks a bit of styling).

I wanted to start the process of publication process before spending more time on polishing the app - in case you don't want it in your store or there are other issues that need solving.

I have a few of questions that I'll humbly ask as part of this PR:

1. How can I make my app dependant on the photos app? (Is there a clever way to not allow installing the app unless photos is available?)
2. Currently I generate my own links by prefixing "/index.php/apps/photoframe/" to each link. Should I be doing something different?
3. Photo Frame generates a public link for each photo frame (a non-guessable link with a 64 char alphanumeric token). The URL only lets visitors fetch the current photo, so it's only possible to fetch 1-24 photos per day (depending on the photo frame's setting). Nextcloud has a global settings page for sharing. Should my photo frame app adhere to those settings? And if so, which setting and how do I get the current setting?
4. Must I add a database.xml?

[JonathanTreffler]

How can I make my app dependant on the photos app? (Is there a clever way to not allow installing the app unless photos is available?)

I am not aware of any mechanism to make the app store/nextcloud core enforce that dependency. AFAIK you need to check if the photos app is enabled in your php code so any codepaths that call the gallery code cannot be reached (because the api returns an error for example). Ideally this would be a visible error for the admins (for example in the admin instance self checks section).

Currently I generate my own links by prefixing "/index.php/apps/photoframe/" to each link. Should I be doing something different?

Yes. If you want to generate the link in the backend use https://nextcloud-server.netlify.app/classes/ocp-iurlgenerator (see example usage) or in the frontend use https://nextcloud-libraries.github.io/nextcloud-router/ .

Must I add a database.xml?

No, database.xml has not been used in many years, if it is still referenced by any documentation that documentation is outdated.

(I am not a nextcloud employee, but a community member, so I can only answer technical questions not any about app store policies)