process duplicates differently

[tttp]

Hi,

we have around 10% of duplicates, most of them are because they took actions on different widgets (partners), but some are folks that take multiple actions on the same page.

for most of the campaigns, it makes sense to "skip" duplicate actions. it's already the case for petitions, were we silently ignore duplicate signatures and remove them from the total

we have a dupeCount column on actions, we should (option per campaign, on by default):

• send an email "thanks, but dupe", informing the supporters their action isn't counted, and prevent them to take multiple actions in vain. the subject + text is configured on a template, with a default per language
• for some campaigns, block further processing (eg do not send an email to the target, do not send the duplicate to the rabbitmq queues).