[Enhancement] In the feed list, show the categories name associated for each feed

[Massedil]

With 550 feeds, it is very difficult to find a specific feed in the feed list because they are sorted by categories but without section with the name of the categories they are in.

Actual:

Feeds:

- Feed 1a
- Feed 1b
- Feed 1c
- Feed 2a
- Feed 2b
- Feed 2c
- Feed 3a
- Feed 3b
- Feed 3c

Expected:

The categories name, then the feeds name.

Like in the FreshRSS web app.

Feeds:

- Categorie 1
  - Feed 1a
  - Feed 1b
  - Feed 1c
- Categorie 2
  - Feed 2a
  - Feed 2b
  - Feed 2c
- Categorie 3
  - Feed 3a
  - Feed 3b
  - Feed 3c