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
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.

This closes ticket #21060.