Fixed #21235 -- Disabled savepoints for old versions of SQLite.

Thanks Ramiro for the report.
Re-added link to SQLite notes from DB section of install doc.
Fixed #21236 -- Allowed migrations to work with unique_together tuples.

Thanks hjwp for the report.
Fixed #18263 -- Corrrected handling of hidden fields in tabular admin inlines.

Thanks hvdklauw for the report and patch.
Fixed #21241 -- Avoid extraneous JOINs in admin changelist search.
Added a link in admin branding block to admin index.

Refs #13749 and #21220.
Revert "Fixed #21241 -- Avoid extraneous JOINs in admin changelist search."

This reverts commit 698dd82eee1cb83f51d4cd39546461bf76976b5e.

The patch introduced a backward incompatible change.
Added min-width for date and time inputs in admin.

refs #21220.
Cleaned up admin CSS for consistency.

* header selected twice -merged
* match indentation throughout
* color names to hex

refs #21220.