Make static content static.

This was another source of poor performance, particularly generating the
format type advice data structures. Instead of storing govspeak in
locale files and loading/parsing them on every request, we store the
static HTML either directly in the appropriate partial or as a data
structure in an appropriate place.
Remove unnecessary markup.

The "govspeak" divs are superfluous.
Get rid of EditionFormBuilder

The last "magic" edition form builder method left is now
alternative_format_provider_select, which is only ever used in one
partial. By replacing it with plain HTML and the two underlying form
helper method calls, we can get rid of the form builder entirely, which
is one less thing making whitehall a special little snowflake.
Cache alternative format provider fields

For now we can use the same base cache digest as for organisations as
the alternative format email addresses are an attribute on the
Organisation model.
Update document collection fields caching

This makes the document collections select field caching a bit more
efficient and brings it into line with the pattern used for the other
fields.
Cache worldwide organisation fields
DRY-up and memoize cache digest calculations

There's no need to re-calculate the digest during a single request
(which is the context within these helpers will be called), so we can
memoize the calculated digest directly in the helpers.