Feat/collections editing

[MasterKenth]

Managing collections through API and frontend.

TODO:

• Basic CRUD API
• Support URLs (supply in PUT set_collection_files as FormData seems dirty? But can't mix FormData (for binary files) and JSON (more appropriate for URLs) in the same endpoint. Or separate endpoint, but how to merge with existing collection content instead of destroying and recreating entire collection?) Done as FormData for now
• Administration UI
• Secure endpoints
• (?) Move generate_chunks and other parts copy-pasted from upload_and_vectorize_handler into its own thing (VectorService?)