Feed avatar improvements

[facundoolano]

1. There's a CSS fallback for when the image is missing, but it doesn't seem to work in mobile in safari.
2. I tried multiple combinations of the favicon fetching logic, and none picks up the ideal icon for all feeds. It would be best to make the field editable to let the user manually correct when the default icon looks bad. If code doesn't get too complicated, we could display all found options so the user knows what's available
3. It would be better to fetch and store the feed image locally instead of hotlinking.
4. It would be nice to show the feed avatars in the command lists and the autocomplete options, instead of the awesome icon.