users value constraints: needed cleanup and decisions

[gsrohde]

area

• [ ] Decide on constraints
• [ ] Add to migration

salt

• [ ] Eliminate NULLs
• [ ] Add not null constraint

remember_token, remember_token_expires_at

• [ ] Decide what to do with these columns

state_prov, postal_code

• [ ] Decide if any constraints beyond not NULL and whitespace-normalized apply
• [ ] If so, add them

Details

area

This column currently isn't very meaningful. We have a mixture of values like "tundra" with values like "Agriculture" and "Industry".

salt

As mentioned in GH #205, 2 rows have NULL in the salt column.

remember_token, remember_token_expires_at

Both of these columns are always null.

state_prov, postal_code

Since these vary so much from country to country, it seems doubtful we can come up with any meaningful constraints that apply to all countries.

We could, however, have checks for certain countries. For example, something like CHECK (postal_code ~ '^([0-9]{5}(-[0-9]{4})?)?') OR country != 'United States')`. A more complicated constraint that might cover a number of countries could be written using a function. Is this worthwhile?