Improve handling of nested resources

[pablobm]

A common cause for complaint from Administrate users is that nested collections (typically HasMany fields) are limited in several ways. Issues include the following:

• Have a NESTED_ATTRIBUTES configuration to control what is being shown for HasOne relations #1559 show different attributes depending if looking at the model itself, or looking at it as an associated resource (has_many/has_one/belongs_to)
• Has_One field does not respect custom views #1563 templates are different for has_many/index, has_one/show, leading to changes not being propagated
• Pass option to Field::HasOne to only display specific columns #1877 nested dashboards with different details
• Show page with HasMany calls the wrong _collection view for the related class #2049 incorrect templates loaded for associations
• Manual sorting of HasMany children list #2276 custom actions in has_many tables

Topics include:

• List different attributes depending if looking at the resources on their own index page or within a HasMany/HasOne collection
  • For example, if looking at "magazine" which has many "issues", the list of issues should not include a "magazine" column (their BelongsTo) because it's already implied in the page.
• When overriding views, use the correct one for the dashboard of the association, rather than the parent.
  • For example, "magazine" has many "issues" and we override the "index" template of "issues". This will not be shown in the issues list of the magazine page and instead it will use the "index" template of the magazines.
  • Proper handling of this would also allow showing custom action columns within HasMany listing. For example "up" and "down" actions to reorder lists.