Unsubscribe from feed through link or "#"

After importing old OPML files, you may discover that many feeds are
obsolete or uninteresting. You list the feeds entries and determine that
you want to unsubscribe. This needs three clicks (edit feed, delete,
confirm) and requires moving the mouse to hit the different targets.

This quickly becomes tiring, if you are up to possibly deleting hundreds
of feeds. One mediation, introduced in this commit, is to add an
unsubscribe link to each feed's entry listing view, and also adding a
keyboard shortcut.

The keyboard shortcut "#" is:
* longer than one keystroke (requires shift)
* hard to type by accident
* used in Google products (thanks for the hint @fguillot)

In an effort to try to reduce the number of accidental feed
unsubscriptions.