[Backport] Mark more strings translatable

Fixes #1171

(cherry picked from commit 0a72bf27c93322a10afbda602e87935438272552)
[Backport] Correct handling of offer creation without a range

The workaround for JSON serialisation introduced a bug that was
triggered when the offer contained no range, which is allowed for
certain offer types. In that case range is None, and accessing range.id
threw an error.

Fixes #1167. Thanks to @izidormatusov for reporting.

(cherry picked from commit 1e85821d85068c71d7d5b959fce743f1b7bfe248)
Fix issue with custom user model in order migration
Fix migration compatibility with Python 2.6

@mbertheau provided a fix in #1176. That commit has been applied to
master. This is fixing the same issue for Oscar 0.6
Mark some strings translatable
Add release notes for 0.6.3
Ensure order number searches don't error

Fixes #1186
Update release notes
Fix lint error
Update translations for 0.6.3
Bump version to 0.6.3
Merge branch 'releases/0.6'

After 0.6.3 release.

Conflicts:
	docs/source/releases/index.rst
	oscar/__init__.py
	oscar/apps/order/migrations/0019_auto__chg_field_order_billing_address__chg_field_order_user__chg_field.py
	oscar/apps/promotions/models.py
Merge branch 'master' of github.com:tangentlabs/django-oscar