Allow selecting a category in feed add URLs (#9047)

* Support categories in feed add URLs

## Summary

- Honour the `category` query parameter when opening the add-feed form.
- Keep the existing `cat_id` parameter as a backwards-compatible fallback.
- Use `category` for the add-feed links in the subscription view.

This makes the URL proposed in #4160 select the intended category:
`?c=feed&a=add&category=1&url_rss=https://example.com/feed.xml`.

## Validation

- PHP syntax checks for the changed controller and templates.
- Focused request-parameter checks for explicit `category`, legacy `cat_id`, and the default category.

Fixes #4160


* Simplify and small fixes

---------

Co-authored-by: Gerard Alvear <gerard.alvear@logiqd.me>
Co-authored-by: Alexandre Alapetite <alexandre@alapetite.fr>

app/Controllers/feedController.php

@@ -124,6 +124,7 @@ class FreshRSS_feed_Controller extends FreshRSS_ActionController {
 	 * GET request displays a form to add and configure a feed.
 	 * Request parameter is:
 	 *   - url_rss (default: false)
+	 *   - cat_id (default: 1)
 	 *
 	 * POST request adds a feed in database.
 	 * Parameters are: