cubicweb-tag #1553045 tags equivalent syntax in tags box [open]

It would be nice to be able to type in the ajax tags box that two tags are equivalent by typing "tag1 = tag2".

This is inspired by semanticscuttle which uses this syntax

Note: use "=" to make synonym two tags. e.g.: britain=united kingdom

Furthermore, it implements the notion of contains (but this is probably for a different ticket)

use ">" to include one tag in another. e.g.: europe>britain>edinburgh