[1.5.x] Made description of LANGUAGE_CODE setting more clear.

297f5af222 from master.
[1.5.x] Fixed #20944 -- Removed inaccurate statement about View.dispatch().

Backport of bac4d03ce6 from master
[1.5.x] Documentation -- Clarified use of 'view' in test client introduction.

Backport of 2e926b041c from master
[1.5.x] Fixed #20922 -- Allowed customizing the serializer used by contrib.sessions

Added settings.SESSION_SERIALIZER which is the import path of a serializer
to use for sessions.

Thanks apollo13, carljm, shaib, akaariai, charettes, and dstufft for reviews.

Backport of b0ce6fe656 from master
[1.5.x] Typos introduced in 57c82f909b.
[1.5.x] Fixed typo in docs/topics/conditional-view-processing.txt

Backport of 5fd2c979cb from master
[1.5.x] Fixed #20981 -- Noted the default value of disable_existing_loggers.

Backport of 095643e691 from master
[1.5.x] Minor spelling correction in ModelForms docs

Backport of 2fbf949760 from master