Allow Size And Placement Of Fields

[dyoder]

[This might be more of a ticket for Vellum Field.]

Presently, Posh attempts to guess at spanning fields based on the type (basically, text and custom fields get span two columns). While smart defaults are nice, we're inevitably going to want to override them.

Our options for implementing this include:

1. utility classes (ex: wide)
2. attributes (ex: width=2)
3. intrinsic widths on descendants
4. smarter rules

Generally, we treat (1) as a last resort. Option (2) is essentially a variant of (1), and arguably worse: at least CSS is intended to representation presentation. Options (3) and (4) are related, in that (3) is essentially a form of (4). For example, a phone number field could span a single row, while an email field might span two. That encodes knowledge we have about the typical length of email addresses versus phone numbers.