stash: make snapshot message customizable

Add option `magit-stash-snapshot-message-format'
and function `magit-stash-format-snapshot-message'.
magit-stash-index{,-snapshot}: new commands
Merge branch 'n/stash-index' into next
magit-commit-popup: support --author option
magit-key-mode-popup-committing: support --author option
Merge branch 'master' into next

Conflicts:
	magit-key-mode.el