Ability to create tags in the UI/API

[risdenk]

Tags are either administrator defined or created on import. There doesn't seem to be a way in the SMUI UI/API to create tags.

Looking at the other options in the API, there is no PUT/POST for inputTags:

https://github.com/querqy/smui/blob/master/conf/routes

As an end user, it would be great to be able to type in the tags box and if it doesn't exist create the tag. This should only be allowed when there are no predefined tags.