[PD-629] Adds Location model

This creates a `mypartners.models.Location` object and associates the field 'locations' from `mypartners.models.Contact` to it as a `ManyToManyField` named `locations`. The point of this particular PR is simply to ensure that the `add_locations` function, when run, adds a single location to each contact that is a mirror of the current data. Future PRs will then remove the address information and deprecate the related fields as well as making sure that contacts imported from the partner library use locations instead of those address fields. 

I could have excluded the one of function from the PR, but thought it might be best to publish it for examination.