Use "shift-g" as a shortcut to refresh an issue from github

cf #155
Add a "review comments" tab on pull requests

cf #155

Next step will be to remove all the code comments on the "discussion"
(renamed to "discussion & updates") tab, replcing them with simple
informative line with a link to the review tab (and also maybe directly
to the files tab if comment not outdated)
Correct edit/delete rights on code comments

cf #155
Collapse commits groups in discussion tab by default

cf #155
Replace code comments in discuss tab by a note that some were added

cf #155
Add icon for commits/pr-comments groups and change events in issue discuss tab

cf #155
Links from discuss tab to review tab

cf #155