Include migration tasks from gem

@maxfliri
@gtrogers
Replace old migrations with new one for initial setup

@maxfliri
@gtrogers
Convert index on artefacts slug and format to unique

@maxfliri
Remove index declaration from model properties

Without auto migrations, index directives on properties are not used and
can easily become inconsistent with actual indexes defined in the data
schema

@maxfliri
Add index on artefacts.collected_at

@maxfliri
Upgrade to recorder gem v0.3.1:

Datamapper configuration moved to yaml file

@maxfliri
Using gem method to drop indexes

@maxfliri
Removing last references to local datamapper config

@maxfliri
Add comment to first migration.
Removed dubilcate task for db configuration

@maxfliri
Merge pull request #8 from alphagov/db_migrations

Db migrations