This commit adds a new database table user_roles to prepare for the
introduction of a voter management system. All existing enabled voters
are added to the VOTER role.
- implement custom http.Filesystem boardvoting.AssetFS
- replace "footer" and "header" with "footer.html" and "header.html"
- change renderTemplate to use Assets
- use boardvoting.GetAssetFS() with http.Fileserver
- switch from goose to github.com/rubenv/sql-migrate
- move assets (static, templates, migrations) to boardvoting package
- add generated boardvoting/assets.go
- remove unused static files from static directory
- add package db with db migration configuration