Remove annotations from the features section of each element

[srawlins]

Fixes #3265

There is a lot of cruft around the templates but here is the gist:

• In model_element.dart, features used to contain both "features" like covariant, read-only, etc, and annotations. As our annotations have grown, these really should be separated. I remove annotations from features. They should not be included in this collection.
• Fix various templates:
  • Rename _annotations partial to _container_annotations. This partial was only used for containers, which list annotations in their relationships section (implementors etc), and uses different HTML from the other annotation lists.
  • Add a new _annotations partial for non-container annotation lists.
  • In _accessor_getter, _accessor_setter, property, and top_level_property templates, add a render for _annotations.
• Add annotations template render tests for properties, methods, and enums. More testing is needed, but this is a good start for this fix.

[pq]

LGTM!