backport: Fix capitalization on confirmation prompts

Added Backport._prompt_for_approval_or_exit() which will generate a
consistent cancellation message of "Backport cancelled" or "Revert
cancelled". Use in all places whereself.prompt_for_approval_or_exit()
was used.

Before:

Continue with Backport?
Ignore dependencies and continue with Backport?
backport Cancelled

After:

Continue with backport?
Ignore dependencies and continue with backport?
Backport cancelled