elfeed: Add an unjam keybinding

[algernon]

Major reason is that when the connection is flaky, elfeed has trouble timing feed fetches out. Leaving it alone overnight with an hourly auto-update, I usually end up with 3000+ items in the queue and growing. Quick access to elfeed-unjam comes in very handy here.

[syl20bnr]

Thank you 👍
Don't ask why, I felt that gu was better then J to unjam :-) I also updated the documentation and move the binding for force update to gR. Related commit f9bd708
Cherry-picked into develop branch, you can safely delete your branch.