This reverts commit 9440bf47.

When you're reading an entry, you may want to go to the feed first,
before quickly continuing to reading all of the feeds with another "g
f".

- The max size for tsvector is 1 MiB
- We index only the first million of characters,
  it should be enough for most feed entries.

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.

- Rename processor package to filter
- Remove boilerplate code

- Add browser package to handle HTTP errors
- Reduce code duplication

- Avoid doing multiple buffer copies
- Move parser and format detection logic to its own package

This enhancement offer the possibility to open articles in the background by using the browser hotkey Ctrl+Return or Cmd+Return.