Move collection of possible actions to one place

There were lists ['subscribe', 'unsubscribe', 'update'] in couple of
places in the code, move it to one place and change to a tuple.
Add NEWSLETTER_CONFIRM_FORM... settings

Add processing of NEWSLETTER_CONFIRM_FORM setting and
NEWSLETTER_CONFIRM_FORM_<ACTION> settings. Result is stored in
CONFIRM_FORM_ACTION dictionary and can be used to decide whether
activation or particular action (subscribe, unsubscribe, update) needs
to be confirmed by submitting activation form.

Update receiver of setting_changed signal to cover new settings.
Optional skipping of activation form in views

Depending on setting stored in CONFIRM_FORM_ACTION,
UpdateSubscriptionViev will activate subscription, update and
unsubscription requests straight after clicking on activation link,
without additional step of submitting activation form.
Fix test_update_request_activate

It should use update_activate_url and not subscribe_activate_url.
Add confirm form override_settings for tests

Some of existing tests depend on confirm form 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 add override_settings(NEWSLETTER_CONFIRM_FORM_...) to
some tests to make sure each test is run with proper settings.
Add tests for requests without confirmation form

Add test_<action>_request_activate_form_off methods testing context
variables and db changes for subscribe, unsubscribe and update
activation requests with confirmation form switched off in settings.
Document NEWSLETTER_CONFIRM_FORM... settings
Merge pull request #2 from jkuczm/confirmation_form

Optional switching off of activation form