[1.7.x] Bump version number for 1.7 alpha 1.
We should branch after the first beta.

We did this for 1.6 and it was very effective. 95%+ of fixes which merge
during the alpha are backported, as the policy is "all but really major
features". It's easier to just not merge any really major features.
After beta, we have feature freeze so we need to backport bugs to stable
but not features, so then the branch makes sense.
Add docs about converting from South
Fixed some punctuation; thanks Chris Jerdonek.
Fixed #21861 -- Fixed typo in 1.7 release notes.

Thanks marktranchant.
Fixes #21864: Close string in example of custom lookups doc page.
Fixed #21864 -- Fixed missing quote in custom lookup example.

Thanks to Chris Barna for the report.
Merge pull request #2205 from ctbarna/21864-custom-lookup-unclosed-string

Fixes #21864: Close string in example of custom lookups doc page.
Fixed #21847 -- Clarified where ContactForm was defined
We should branch after the first beta.

We did this for 1.6 and it was very effective. 95%+ of fixes which merge
during the alpha are backported, as the policy is "all but really major
features". It's easier to just not merge any really major features.
After beta, we have feature freeze so we need to backport bugs to stable
but not features, so then the branch makes sense.
Merge pull request #2203 from mjtamlyn/post-beta-branching

We should branch after the first beta.
Merge branch 'stable/1.7.X'

We fork at beta. This branch will be recreated when the first beta is created.
It has been merged in to maintain the version number bump and tag.
Fixed #21865 -- Incorrect signs in documented example.

Maths is hard.
Fixed #21787 -- regression in MTI .exclude() queries
Fix soft-apply detection of migrations with dependencies
Fixed #21824 -- Added reference to LTS in docs/internals/security.txt
Fixed #21869 -- Fixed docs building with Sphinx 1.2.1.

Thanks tragiclifestories for the report.
Fixed #21870 -- Admin check for list_editable_item

During the admin check for list_editable _check_list_editable_item
should return an empty list if all checks pass. Additionally the
Testcase test_readonly_and_editable was changed to test what the
name implies instead of duplicating the logic of test_readonly.
Fixed typo in error message.
Fixed #21867 -- Removed AppStaticStorage; app paths are now AppConfig's job.

AppStaticStorage only provided one thing over FileSystemStorage, which was
taking an app name (import path) and translating it into a filesystem
path. This is now something that should be done via app_config.path instead,
leaving AppStaticStorage with no reason for existence. It should be safe to
remove, as it was undocumented internal API.

There was some kind of feature in the AppDirectoriesFinder code related to a
"prefix" attribute on the storage class used by AppDirectoriesFinder. Since
this feature was undocumented, untested, and of unclear purpose, I removed it
as well.