Allow users to merge accounts

[Amenadiel]

Since there are mods to allow users to delete themselves, I guess it shouldn't be too hard to code something to allow users to merge their accounts when they have several ones.

I've come to this need after merging with (or assimilating) smaller communities, in which some of my users where already registered with the same nick and different email adresses. The result is a lot of imported_whoever nicks, and the original owners tend to just abandon one of the accounts instead of asking for me to merge them

I guess that an email confirmation for two accounts could confirm they belong to the same owner, and then proceed with the merge on the background.