Add settings proxy class

Adapt RICHTEXT_WIDGET setting.
Add NEWSLETTER_CONFIRM_EMAIL... settings

Add processing of NEWSLETTER_CONFIRM_EMAIL setting and
NEWSLETTER_CONFIRM_EMAIL_<ACTION> settings.
settings.newsletter_settings.CONFIRM_EMAIL_<ACTION> 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 CONFIRM_EMAIL_<ACTION> settings, <Action>RequestViews will
subscribe, update and unsubscribe user without email confirmation.
Adjust templates to skipping email confirmation
Document NEWSLETTER_CONFIRM_EMAIL... settings
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.
Move update_subscription function to Subscription.
Make subscribe() and unsubscribe() internals.

These methods should not ever be called from other
places than the current save() method, hence mark
them as internal.
Comment about reworking save().
Small syntactic cleanups.
Factored out generic components of settings.

This might be a pattern that comes in handy for
later apps.