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.

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• There is no breaking changes
• I really tested my changes and there is no regression
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Fixes #375