Fixed #20920 -- Consolidated F() and Q() documentation

AdminSite now has overridable site_header, site_title and index_title attributes. Changed
each admin view to pass these to the context (in a new AdminSite.each_context() method).
The intent here is to make it easier to override these things in the common case, instead of
having to override a template, which is a bigger burden.

Related Fields documentation improvements

Thanks EvilDMP for the report.

documented RelatedManager calling save() behind the scenes, added section on direct replacement of related object set

Thanks malte for the report.

Added /build/ to .gitignore, so it doesn't accidentally get committed

Thanks jdunck for the suggestion.

Fixed #21015 -- Fixed MigrationLoader when importlib.import_module returns a file module or an empty directory.

The documentation and comments now all use 'an' to
refer to the word SQL and not 'a'.

1. Moved common logic between assertContains and assertNotContains
   into a separate function _assert_contains()

2. Moved common logic between assertTemplateUsed and
   assertTemplateNotUsed into a separate function
   _assert_templateused()

Thanks Marc Tamlyn for the report and review.

Removed "makemigrations --force" from docs since it doesn't actually exist.

Thanks Keryn Knight for the suggestion.

Thanks tuxcanfly.

refs [fb1dd6b1] and [1c4a9bd9]

exc_value might be None even though there's an exception, at least on
Python 2.6. Thanks Thomas Chaumeny for the report.

Fixed #21034.

Forward-port of a8624b22 from 1.6.x.

Thanks marky1991.