[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
Introduced 'mincount' field for settings.OSCAR_SEARCH_FACETS instead of 'show_zero' to match solr's config values. This parameter allows to include zero-counted facets in search result.  Default value is 0. Also 'limit' field added to fix default Solr behavior which limits max facets count to 100. Negative value means no limits.

Conflicts:

	sites/demo/settings.py
Merge branch 'releases/0.6' of https://github.com/tangentlabs/django-oscar into projects/butik-shop
Restore setting in demo