Add sqldropindexes to manage

Change patch from https://code.djangoproject.com/ticket/5568
to work on modern Django.
Add special case for MySQL which has different syntax for DROP INDEX.
Add unit tests for the new functionality.
Made changes asked in review by HonzaKral

Add documentation for new command in django-admin.
Merge pull request #817 from rybaktomasz/ticket_5568

Fixes #5568 -- DROP INDEX subcommand