Add NEWSLETTER_CONFIRM_EMAIL... settings

Add processing of NEWSLETTER_CONFIRM_EMAIL setting and
NEWSLETTER_CONFIRM_EMAIL_<ACTION> settings. Result is stored in
CONFIRM_EMAIL dictionary and can be used to decide whether confirmation
email is needed for particular action (subscribe, unsubscribe, update).
Optional skipping of email confirmation in views

Depending on setting stored in CONFIRM_EMAIL, <Action>RequestViews will
subscribe, update and unsubscribe user without email confirmation.
Adjust templates to skipping email confirmation
Document NEWSLETTER_CONFIRM_EMAIL... settings
Add signal receiver reseting CONFIRM_EMAIL...

Create update_confirm_email function that resets settings related to
email confirmation and connect it to setting_changed signal. It's needed
in tests for override_settings to work.
Add override_settings for existing tests

Some of existing tests depend on confirm email settings being set to
default values. Since in test_settings those setting are not present
default values are used and travis tests pass, but if developer would
run tests in his/hers project with non-default values of settings tests
would fail. So added override_settings to make sure each test is run
with proper settings.
Add testing of action_done in template context

Subclasses of ActionRequestView now should pass additional variable
action_done to template context. This variable should be false if email
confirmation is on because actions can not be done in
<Action>RequestView.
Add tests for requests without email confirm.

Add test_<action>_request_post_no_email methods testing:
* response status
* context variables
* db changes
* mail outbox
for subscribe, unsubscribe and update requests with email confirmation
switched off.