Documentation -- Improved description of cache arguments

- Fixed some grammar and formatting mistakes
- Added the type and default for CULL_FREQUENCY
- Made the note on culling the entire cache more precise. (It's actually
  slower on the filesystem backend.)
Fixed #20887 -- Added a warning to GzipMiddleware in light of BREACH.

Thanks EvilDMP for the report and Russell Keith-Magee
for the draft text.
Fixed #21056 -- AdminSite.app_index no longer blindly accepts any app-labelish input.
Fixed #21089 -- Allow TransactionTestcase subclasses to define an empty list of fixtures.

Thanks to lgs for the report and initial patch.
Fixed #21090 -- Allowed backends to provide dotted field path to inspectdb.
Minor typo fix in django.contrib.auth.models.User docs
Merge pull request #1618 from pcx/minor_typo_fix

Minor typo fix in django.contrib.auth.models.User docs
Add a test for the geo-enabled inspectdb command
Fixed inspectdb test for spatialite >=4
Fixed #21099 - Skip DistinctOnTests unless backend can_distinct_on_fields
Merge pull request #1622 from manfre/ticket-21099

Fixed #21099 - Skip DistinctOnTests unless backend can_distinct_on_field...
Fixed a couple of typos in GeoDjango docs.
Fixed #21095 -- Documented new requirement for dates lookups.

Day, month, and week_day lookups now require time zone definitions in the database.
Fixed #20844 -- Made AdminEmailHandler respect LOGGING 'formatter' setting.
Fixed #12288 -- Validated that app names in INSTALLED_APPS are unique
Documentation -- added instructions on working with pull requests

Since non-core contributors are asked to review patches, instructions
on working with pull requests were added to the Working with Git and
GitHub page (based on the existing instructions in the core
committers page).
Fixed #21094 -- Updated reuseable apps tutorial to use pip for installation.

Thanks ylb415 at gmail.com for the suggestion.
Fixed #21100 -- Noted that Create/UpdateViews.fields is new in 1.6

Thanks AndrewIngram for the suggestion.
Fixed #21101 -- Updated urlize documentation to mention email addresses
Fixed #21060 -- Refactored admin's autodiscover method to make it reusable.

We want to be able to use it for instance for discovering `tasks.py` modules
inside the INSTALLED_APPS.

This commit therefore moves the logic to `autodiscover_modules` method in
django.utils.module_loading.