Make DatabaseCollation.supports? check database encoding

[lizconlan]

The DatabaseCollation.supports? method only checks whether a named collation is available to the current database, it does not check whether it is compatible with the database encoding.

This is currently preventing us from using the it_IT locale as this does exist in the database but is only available for UTF8 encoding (our database uses SQL_ASCII). If we try to use the incompatible location after having checked its availability, we get an error back from Postgres which prevents PublicBody#list from rendering:

ERROR:  collation "it_IT" for encoding "SQL_ASCII" does not exist

(Background - the lookup on pg_collation is intended to be a multi-part key which includes the encoding - ref: https://www.postgresql.org/docs/9.2/static/catalog-pg-collation.html)

Fixes mysociety/dirittodisapere-theme#106

[lizconlan]

Useful things:

SHOW server_encoding; retrieves the database encoding
SELECT pg_encoding_to_char(collencoding) FROM pg_collation where collname = 'it_IT'; gets a comparable encoding string for the named collation

(alternatively a collencoding of -1 is compatible with everything so that would also be fine - pg_encoding_to_char returns a blank string)

[lizconlan]

merged as hotfix/0.27.04 and hotfix/0.26.0.9