Feature Request: Update email address for all subscriptions associated with subscriber

[raamdev]

If a subscriber wants to change their email address for all of the subscription they've created your site, there should be an easy way to do that. If someone has a hundred Comment Mail subscriptions on your site, there's currently no easy way to change their email address for all subscriptions at once (as of v161213), either from the front-end or from the backend.

At this time, the only option would be to produce a CSV export (with the Pro version), search/replace for the email address in the export file, and then re-import the changes. However, if someone has 30,000+ subscriptions, that's not really feasible as you'd need to produce multiple export files (each limited to 1000 rows) and then search/replace in each one.

A feature on the backend with two input boxes, one for the old email address and one for the new email address, that will search/replace and update all of the subscriptions associated with the old email address at once, would be great. A front-end feature to allow subscribers to do this themselves would also be great.

Somewhat related to #327.