migrations doesnt create {text|varchar}_pattern_ops for postgres indexes

[adityagupta104]

This is an issue which was raised a looong time ago and was fixed for syncdb here - #12234 and #19441.
Django 1.7.1 doesnt add these additional indexes for postgres when using migrations. duh.

[Baptiste Mispelon]

Hi,

I can indeed reproduce this on 1.7 and master.
It seems that tests were added for the varchar_patter_ops after #12234 [1] but they still pass (probably because they don't use the same codepath that the migrations framework uses).

This might be a serious performance issue so I'm upping the severity to "release blocker".

Thanks!

[1] ​https://github.com/django/django/blob/master/tests/indexes/tests.py

[Tim Graham]

Claude has done some work in moving more DatabaseCreation functionality to the schema editor in ​PR 3220. I've asked if he might be able to create a patch that would be appropriate for backporting based on that work.

[Claude Paroz <claude@…>]

In 8d7a48027ef6848f693f57ae442a1b0ace0ebcdc:

Fixed #23954 -- Added special text/varchar PostgreSQL indexes in migrations

Thanks adityagupta104 for the report and Tim Graham for the review.

[Claude Paroz <claude@…>]

In 6ba960266a14756dbf6365029d46bd157bd2b155:

[1.7.x] Fixed #23954 -- Added special text/varchar PostgreSQL indexes in migrations

Thanks adityagupta104 for the report and Tim Graham for the review.
Backport of 8d7a48027e from master.