Update WS api/issues/tags to work with new rule index types

The web service api/issues/tags relies on the rules index. Because the tags of the rules index will be split (system tags vs. organization specific custom tags), this webservice will have to be updated.

Making the web service organization specific will be covered in another ticket.

Keep in mind, that the webservice returns tag suggestions from these locations:

• tags on other issues
• system tags of rules
• custom tags of rules