Allow Expressions when saving new models

[Alex Hill]

Opening a ticket after discussion on IRC with Josh.

You can use Expression when updating models, but it would also be useful to be able to use them when creating models, particularly in order to apply database functions to newly-inserted values.

My specific use case for this is a PostgreSQL TSVectorField, where instead of just inserting a value, I want to insert strip(to_tsvector(<ts_config>, unaccent(<value>))). Ideally, I would be able to define these all as Funcs and insert an Expression.

I propose to fix this by making SQLInsertCompiler check for resolve_expressions and as_sql, as SQLUpdateCompiler does.

Alex