Fix dealer registration

Applies a new round of fixes to finalize dealer registration for AC.
I'm guessing that you're just using the "website" column for the tax id since it's a column that already exists and therefore you don't need to do a schema change?

In general if you ever find yourself wanted to do that, then I recommend changing the name of the column variable.  So instead of

```python
class Group(MagModel):
    ...
    website = Column(UnicodeText)
```

you could say

```python
class Group(MagModel):
    ...
    tax_id = Column('website', UnicodeText)
```

See http://docs.sqlalchemy.org/en/rel_0_9/orm/mapper_config.html#naming-columns-distinctly-from-attribute-names for the docs on that; basically it would let you keep using the "website" column in the database (and thus avoid needing to do any kind of schema change) without having to confusingly refer to ``group.website`` every time you want to refer to the tax id in your code.

In fairness, I'm not 100% sure this would work with e.g. the ``session.group()`` method, since I didn't think about this edge case when I was writing those methods, but it's worth a shot.  Or not, if it seems like too much trouble :)