[1.7.x] Added missing spaces to implicitly joined strings

Backport of 4c39c270af from master
[1.7.x] Added spaces to message from 4c39c270af91ddbc213e077fc06b4bf67c7c6e99.

Backport of add78c58b5 from master
[1.7.x] Fixed docs/ref/contrib/auth.txt so that sphinx links work properly.

Backport of 5359188b08 from master
[1.7.x] Removed reference to old middleware

Backport of 0b5bafe993 from master
[1.7.x] Fixed #21942 -- Moved Form.clean() to form API docs.

Thanks cjerdonek for the suggestion.

Backport of 874053edf9 from master
[1.7.x] Fixed #19489 -- Documented host parameter of assertRedirects().

Thanks mrknacky at gmail.com for the report and gajimenezmaggiora
for the draft patch.

Backport of e3c7af18a3 from master
[1.7.x] Fixed #22482 -- Recommended using data migrations in contrib.sites docs.

Thanks scibi for the report.

Backport of bbf0a9545b from master