Feature summary: Privileged user (admin etc.) should be able to reset other user's signature if their signature content violate local policy.

Use case:
Many projects have local policy of signature, these policies focus on signature's content and syntax, like length limitation, should not impersonating other's signature, should not be used for canvassing etc. Most policies allow admin to block a user with problematic signature until they change their signature, but the impact of such block may be too big.
Similar to partial block function, which was launched to "prevent damage or disruption to Wikipedia, not to punish users" (WP:PB@enwiki), I thought it's reasonable to develop a function which allow admin (or other privileged user) to reset signature to default status.

Benefits:
This allows contributor continue their contribution without such "punishment", and local signature policy get a better and quicker enforce.