Basic handling of shared recipes

[Murazaki]

Topic and Scope

This PR is meant to allow for shared recipes handling.

It manages the separation of the Recipe folder into two parts :

• the user's recipes, defaulting to "{UserFolder}/My Recipes" folder
• the recipes shared to the user, defaulting to "{UserFolder}/Shared Recipes/{User Id}" folder

Concerns/issues

This PR does need careful handling on the migration part, and will need further test cases.

Formal requirements

There are some formal requirements that should be satisfied. Please mark those by checking the corresponding box.

• I did check that the app can still be opened and does not throw any browser logs
• I created tests for newly added PHP code (check this if no PHP changes were made)
• I updated the OpenAPI specs and added an entry to the API changelog (check if API was not modified)
• I notified the matrix channel if I introduced an API change