This will no longer be used for deployments, as all
service templates have been migrated to clusters.

Aligns with the other reactive cache options by
providing a default that can be overridden if necessary.

It's not needed anymore as we require `#cache_key` instead.

This commit also includes some changes in specs to use
`Class.new` approach.

This allows using `CacheMarkdownField` for models that are not backed
by ActiveRecord.

When the including class inherits `ActiveRecord::Base` we include
`Gitlab::MarkdownCache::ActiveRecord::Extension`. This will cause the
markdown fields to be rendered and the generated HTML stored in a
`<field>_html` attribute on the record. We also store the version
used for generating the markdown.

All other classes that include this model will include the
`Gitlab::MarkdownCache::Redis::Extension`. This add the `<field>_html`
attributes to that model and will generate the html in them. The
generated HTML will be cached in redis under the key
`markdown_cache:<class>:<id>`. The class this included in must
therefore respond to `id`.

Port: EE adjust burndown chart opened issues count

See merge request gitlab-org/gitlab-ce!28777

* Adjust opened issues count in the milestone burndown chart
to match current opened issues in the milestone.
* Fix completion rate calculation to include milestones only.

This turns Notable::RESOLVABLE_TYPES into a
`Notable.resolvable_types`. That allows us to override it in EE.

Refactor