Remove url-sensitive characters prefixing tags

[jaromil]

If a tag is created with a character that affects urls, it fails to be passed as argument.
For instance if a tag is created with a prepended hash (i.e. #tag) then the url listing tag transactions will not be valid, as the browser will take it as an in-page anchor instead of passing it through the http pipeline (i.e. http://localhost:8000/tags/#tag).
Upon tag creation the string should be sanitized by removing all extended url characters.