Add models to track grouping of document series

This commit also contains a migration to copy data from the
document_series_memberships table into the new tables, and a pending
data migration to drop the document_series_memberships table once the
code is running successfully in production.

When the groups are retrieved from a series, they're returned in the
order in which they were added. Similarly for the documents in a group.
Associate editions with a doc series via groups
Rename private method to improve clarity
Use 'Documents' as default heading for doc series groups
Display doc series groups with published docs
Speed up doc series page with SQL-power

Only load the doc series groups from the database that have a
published edition.

Use `includes()` to avoid a couple of N+1 queries.

Total query count to load a page with ~20 docs in the series is down
from around 60 to 13. Page load time is halved in the dev VM.