(#280) Strip whitespace from venue fields

Prior to this commit, Calagator would allow venues to have leading or
trailing spaces persisted in the database, which would lead to venues
having postal codes such as `'97209 '`, thereby reducing the likelihood of
finding duplicates. This commit forcibly strips whitespace from venue
fields during validation by adding `strip_whitespace!` to the
ActiveRecord DSL, technically in the `before_validation` stage.