Adds require serial as Asset Model option

[Godmartinz]

This gives users the option to require serials by asset model.
This can be done via each model individually, or through bulk editing models
Model edit:

Bulk Model edit:

I was concerned with "What if you update an asset model to require serials, how do we deal with that retroactively?" My solution is not to make validation fail if you update an asset w/o a serial, but to provide a 'warning' message that that asset needs a serial appended. it is an undertaking that a user should be prepared to fill.

Asset Update:

(If a bad user deletes the serial, that info is readily available in the history log of the asset.)

Moving forward with asset creation, if the Asset model selected requires a serial, a form validation will fire:

Require Serial Number is also a sortable field in the Asset Models index:

Tests have been included.

[uberbrady]

Probably have a few things to talk about in there, but this is very clean code, very well-executed. Let's chat more offline and see if we want to embed in other stuff or take this as-is.