Improved English in release notes for runserver.

Thanks Tim.
Added more tests and documentation for dictsort.

It's possible to use something like {{ foo|dictsort:'bar.baz' }}
but this wasn't tested or documented.
Fixed a documentation typo introduced by b914991b37.
Fixed #21387 -- Merge two very similar help texts.
Fixed another typo introduced by b914991b3705cb6c91013d962c55cda9deb18d83.

Thanks Claude for catching it.
Fixed typos in documentation.
Distinguish between browser-deprecated locales and internally
deprecated ones.

Thanks to bouke for pointing this out.
Massive migration optimiser improvements + RenameModel opn
Allow squashing of squashed migrations
Added missing info to the release checklist.
Removed "UNDER DEVELOPMENT" header on the 1.6 release notes.
Added more steps to the release checklist.