Rename fields to tags

[Plsr]

Fields should be tags, as they are closer to the concept I would like to use them.

This has to be done in two steps (well, it doesn't have to really, as I am the only user and do not have any data in the application, but I'd like to do it none the less as practice):

1. Create the new tags model, write a tasks that copies the data and deploy all that
2. Delete the old fields model and use tags everywhere where fields where used before.