Allow the addition of Dashboard context to any layout

Description of the need

Currently, the Dashboard context is automatically added to layouts when the path is admin/dashboard. This allows the layout to access Dashboard-specific blocks.

It'd be great to be able to add this context to layouts with other paths. Currently this context is not listed among the options for other paths.

Proposed solution

Be able to manually add the Dashboard context to layouts with paths other than admin/dashboard. This would allow other layouts to access dashboard-specific blocks.

Alternatives that have been considered

None

Additional information

According to a Zulip comment by @docwilmot:

In layout.context.admin there is this line if (!in_array($key, array('string', 'overrides_path', 'dashboard'))) which prevents this ...There isn't any real reason not to be able to add dashboard contexts. We need an issue to remove that exception.

Draft of feature description for Press Release (1 paragraph at most)

Backdrop now includes the ability to add the Dashboard context to layouts with any paths.