Add collections for grouping and sharing entries

I propose adding the possibility to group entries into collections, similar to Zenodo communities. This would make it easy to create and share a curated set of entries, e.g. for projects or working groups.

In a first step, I would only add the possibility for the uploader to assign the entry to one or more collections. On the browse page, a filter dropdown (similar to the existing categories filter) would allow users to pre-filter the entries on eln.community. Sharing the collection can be done by enabling the filter via a URL parameter, e.g. with some syntax like https://eln.community/browse?collection=16.

Adding additional functionality, like allowing others to submit an entry to a collection and allowing a curator to accept or reject entries, can be implemented later on if required.

Start with the browse page and its existing categories filter, then trace how uploaders assign metadata to entries. Define the collection data and assignment flow, add collection filtering on the browse page, and support sharing through the proposed URL parameter. Done means entries can be assigned to one or more collections and the browse filter reliably shows the selected collection.