stock-replies: Add tags with a stock reply

Currently the stock replies script can modify the importance, status, assignee, and package in addition to the comment. It would be helpful if it also allowed modification of the tags for the bug report.

The server team's standard responses include a few that add tags, namely 'needs-upstream-report' and 'bitesize'. I expect there are more such tags that would be worth having.

Tags are a bit challenging for this script because they're applied separately (asynchronously) from the form submission for the other elements. They would need some different mechanism to apply them, perhaps something derived from the buttontags script?