Add move to folder feature

[ahaltindis]

I've been looking for a 'perfect' self hosted gallery app for some time, and this is the best I could find so far. So thanks for this project!

While I am using this app though, I notice couple of features fell short, which are important for my use-case. So moving multiple photos from timeline and folders to another folder is the one I need at the moment (effectively fixes #204 - only move part though). So I decided to implement this. I understand that if this won't be prioritized (I'll use my fork anyway 😄) or cancelled all together.

Couple of notes:

• Webdav natively supports MOVE, so just using it as similar to Nextcloud files.
• If the target exists, it'll fail. (again, same behavior with Nextcloud files).
• Not enabling move in albums and archive as it would probably be doing something that user didn't intend.
• Not adding move operation to context menu of photo view as that doesn't seem very useful (at least to me) so didn't want to add unnecessary noise.
• Using very similar approaches to adding to album and deleting photos.

[gymnae]

Thanks for creating a PR for this, just today I was hoping this feature would come :)

[pulsejet]

Very nice (one of the rare times I've had a PR where I changed nothing 😄).

Of course this will break when the user has e.g. a shared folder or something as their timeline root. But that breaks a lot of things anyway, so wouldn't bother with it for now. Ideally we need the permissions flags for each file so we can make checks on the UI side, but not sure how feasible this is, peformance-wise.