Is there a way to purge unused tags from the DB?

If I miss-tagged a question with "perview" instead of "preview". I go back and fix the tag.

However, I now have a tag "perview" in the system.
Is there a way to purge those unused tags? It would be ok if I had to explicitly do it.

something like: "python manage.py purge_tag perview"

If the in tag was used, I should get an error:

 "Error: Tag 'perview' is in use (in 2 questions: [148, 172])"

Showing the # of questions and the question number would be a nice bonus.

added management commands:

delete_unused_tags (no arguments)
rename_tags (arguments: --from, --to, [--user-id])
rename_tags_id (same arguments as in rename_tags, but renames by tag id's)

Both --from and --to parameters can have multiple tags, so you can split, rename and merge tags. If a parameter has multiple tags, it must be quoted.

Example:

python manage.py rename_tags --from biochemistry --to 'biology chemistry'

To find out more, type python manage.py <command> --help.

Give it a try. I had some dilemma on whether to mark retagging activity as the latest activity on the question or not. Right now the effective user will be marked as having performed the operation.

Please note that with MySQL in askbot transactions don't work, so it will be a good idea to make a backup, at least till you become comfortable with the commands.