[Feature request] Add possibility to merge tags

[DBinaghi]

Sometime it happens two or more tags are added, maybe just differing by one symbol (e.g.: "colored" and "coloured") or by capital/lowercase letters.
At the moment, one has to reassign correct tags, deleting then the unused ones, since admin ui doesn't let assign a tag that already exist.
It would be therefore very useful to have a function that allows merging of tags: by changing one, if that tag is already existing, all tags are reassociated to the existing one.

One possible solution is in #902 and #901

[CharlesButcher]

+1. This must be a very common need.