Allow multiple categories per feed

[darkdragon-001]

Allow multiple categories per feed. Motivation can be found in #375.

• Add a new table for the multiple-to-multiple relation and migrate existing data
• Use a list of categories everywhere in the code
• Tested main UI windows and OPML import

TODO

• UI feed creation and edit need another interface to select multiple categories.
• Can we simplify some queries? With an ORM such a change would have been so much easier.
• Decide when to show categories in the UI. Showing multiple categories for every article clutters the view and distracts from the article. So I tend towards removing them from the lists views and maybe also for single feeds/entries. This will make the UI, the queries and the code much simpler, which is a goal of this project.

Do you follow the guidelines?

• I have tested my changes
• There is no breaking changes
• I really tested my changes and there is no regression
• Ideally, my commit messages follow the Conventional Commits specification
• I read this document: https://miniflux.app/faq.html#pull-request

Fixes #375

[jvoisin]

@darkdragon-001, any plans to come back to this, or can we close it?

[darkdragon-001]

@darkdragon-001, any plans to come back to this, or can we close it?

@jvoisin Thanks for checking. Are you a maintainer of this project? Yes, I still plan to work on this. Some preliminary feedback would help, especially around what would be needed to get this merged.

[mtsz-pl]

I'm also interested in moving this forward. @darkdragon-001 did you try syncing this with current main branch?