Add custom RSS feeds for users

[GeorgeApos]

This PR adds support for user-defined RSS feeds in UniNews. Users can now add their own RSS sources from the app, which are saved locally in the application settings and loaded together with the built-in feeds during refresh.

Fixes #4

[TolisSth]

I pulled the branch locally and it does not run spitting out the following error:

env ❯ uninews
Traceback (most recent call last):
  File "/home/tolis/Documents/CS/UniEcosystem/UniNews/env/bin/uninews", line 3, in <module>
    from main import main
  File "/home/tolis/Documents/CS/UniEcosystem/UniNews/main.py", line 5, in <module>
    from ui.main_window import UniNewsWindow
  File "/home/tolis/Documents/CS/UniEcosystem/UniNews/ui/main_window.py", line 23, in <module>
    from feed_config import get_all_feeds
ModuleNotFoundError: No module named 'feed_config'

Is there any known reason for that, is this something on my end can we clarify this ?