Context Navigation

Changes between Initial Version and Version 1 of Ticket #24234, comment 4

initial	v1
7	7	* it's independently useful (as in my case) outside of `Template.render`
8	8	* it existed in standalone form before 1.8 in the form of `TemplateResponse.resolve_context`, which might be used by others than me. Putting it in a separate function makes for a straightforward migration.
	9	* edit: well-commented copy-paste will be a fine alternative but doesn't quite capture "create a `Context` exactly as Django does" as clearly and succinctly.
9	10
10	11	Would love it if you could be swayed to a +0.