This is the first sketch of a dashboard generation service+domain
implementation in LTD Keeper itself.

- BuiltinTemplateProvider coordinates built-in templates; when we
  implement S3-based templates we'll create an S3TemplateProvider with a
  similar API.
- Context provides dataclass instances that provide information for the
  dashboard that are adapted from the DB models.

We're using ltd-conveyor's s3 functions for uploading objects and
directories.

This fixes some upload issues we found for individual objects into the
bucket.

You can now see the dashboard built with mock data in dashboard_dev/ by
running the test_dashboard_template.py module. This enables you to
quickly refine the built-in template.

Normally main editions should be marked with EditionKind.main, however
if they aren't these extra tests ensure they don't appear in the
releases or drafts lists.