Allow on-the-fly creation of new tags when creating jobs, agents

[guidow]

The admin interface and the REST-API should support creating new tags on the fly when creating new agents or jobs, automatically associating these new agents or jobs with those new tags.

This is how I - and probably many others - have come to think of using tags. Requiring separate steps for creating tags and the entities they describe will probably be a hurdle for te acceptance and usage of tags.

[opalmer]

No argument here. Though I have not looked to deeply into it, using sqlalchemy's events would probably be one way to do this. If that does not work we could try the constructor to the tag class instead.