Add ability to merge two user accounts

It would be super helpful if we could merge two user accounts into one, akin to how we can merge duplicate posts into one.

We started using Featurebase with our site core users before fully integrating Featurebase with SSO in our site. The integration is now complete and our core users who started early ended up with two user accounts in Featurebase - their SSO user and the user they had previously created. Our users are asking if they have to go click through every post they had previously followed with their new SSO accounts and we’d love to not need them to do that.