add slug for Votation and Act, and use it for urls

see #538
add support for slugs in urls

see #538
add a script to synchronize Votation cache with ChargeVote instances

see issue 47 in om-senigallia
modify slug definition for Act and Votation

see #538
add support for Speech slugs

see #538
refine migration

now migrations check that data has unique configurations before introducing
actual UNIQUE constraints in the db.

also, sets default slugs for acts, speeches and votations
Merge branch 'dev-senigallia' of github.com:openpolis/open_municipio into dev-senigallia