Change message templates select order

Message template overrides for specific newsletters were never used,
because "global" message templates were always loaded first.
Move common email tests to a mixin

* Create AllEmailsTestsMixin performing tests for all message types.
* Adapt HtmlEmailsTestCase and TextOnlyEmailsTestCase to inheritance
from AllEmailsTestsMixin.
Customize Newsletter creation in MailingTestCase

* Add get_newsletter_kwargs method to MailingTestCase.
* Adapt MailingTestCase.setUp to use
MailingTestCase.get_newsletter_kwargs.
* Adapt HtmlEmailsTestCase and TextOnlyEmailsTestCase to use
get_newsletter_kwargs.
Remove MailingTestCase.fixtures

It contained only 'default_templates', fixture that was removed in
30d7aaa.
Add test template overrides
Add tests for templates overrides

* Add tests.utils.template_exists function.
* Add assertEmailAlternativeBodyContains assertion.
* Add TemplateOverridesTestCase testing template overrides for subject,
text and HTML body for all message types. Skipped if templates overrides
don't exist.
Merge pull request #66 from jkuczm/fix_message_templates_select_order

Fix message templates select order