Solidus: Time-aware tax rates

[mamhoff]

Tax rates are currently valid "forever". If a tax rate changes in a jurisdiction a store sells to, the tax rate records have to be updated exactly when the new tax rates comes into effect. This is not very user-friendly.

It would instead be nice if we could set a valid_from and valid_until date on tax rates, and have them apply when the current datetime is within these two dates. If both are nil, tax rates can still be indefinitely valid.

[vladstoick]

I will start working on this

[mtylty]

@vladstoick I'm working on this as well, would you mind splitting the work? We could work on solidus_core and solidus_backend separately.

[vladstoick]

@mtylty I think this one might be easier if just one person works on it so I can let you work on it if you already started working