Migrate Tag model

[dhamaniasad]

It seems that when Crestify was published publicly, the Tag model had some VCS issues and we ended up with two different versions of the Tag model in different parts of the code.

We use a M2M relationship, and used an array of strings earlier. We need to add the M2M model back in with a migration, else imports, among other things, will be broken. Issue #33 is also because of this.