Word-of-mouth is a great way to get high-quality leads for cheap.
This addon allows users to create templates for partners to share a link
with their a cover image tailored to them on social media.

A good use case is to allow presenters to advertize their track
during an event, each of them can advertize their own track and the
event in a way that's consistent accross all presenters of the event.

This also allows for rewarding partners who do share the event by
giving them any link once they share the event, to a coupon for example.

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### Open Graph

To get social networks to render the image, we use opengraph with a redirect url.
When a social network bot is detected on the redirection endpoint, the endpoint
serves a minimal html page containing only open graph data. When a regular user
then clicks the link they'll be redirected normally to a url of our choosing.

The list of network bot user agents needs to be kept up to date for this feature
to work properly, and the feature will only work for listed social networks.

### Rendering

The rendering is in two steps:
- ir.qweb to get an html web page
- wkhtmltoimage to actually render the image to a jpeg

wkhtmltoimage works very similarly to wkhtmltopdf, instanciating a headless
webkit engine to render the page. For our purposes the html body should be
self-sufficient, meaning wkhtmltopdf may not execute scripts or fetch data.

### Card Campaign

A card campaign comprises:

- A template, defining the general design/layout of the card in html
- Elements, modifiable by the user to either set static or dynamic values
- A target model, defining what model the dynamic values are picked from
- A preview record, allowing users to preview the card with specific values
- A send button, allowing users to send the card by mail using a domain

Each campaign tracks cards sent, cards opened, cards shared and how many clicks
these cards from social media globally.

The campaign elements simply hold values to be passed to ir.qweb at the time
of rendering. Either a static value (text/uploaded image), or a field path.
The field path is validated based on hard-coded values on each model.
Other field paths may only be set by system administrators, as they may
pose security concerns. **All paths are fetched as sudo**.

Sharing the card by other means than the built-in composer can be done
by just calling `_get_preview_url_from_res_id` on a specific card campaign
for any automated action and/or mail template. The card will be auto-generated
no matter what. Although it is recommended to create it uppon sending
for tracking purposes.

### Card Templates

Templates define the layout of the card in html as well as some
default color and background settings. For the base data the bodies
simply call views which makes it easier to reuse CSS classes and
qweb variables.

### Cards

Cards cache the resulting card image for any recipient so as to prevent compute
spamming, as rendering is relatively expensive. For the same reason card URLs
are hmac-signed so users cannot generate cards for all recipients at once
by just guessing url parameters.

One a user has shared the card, they'll get a message on the preview page
which can be used for them to get small rewards. The message is the same
for every participant, and the "share" detection mechanism is not totally
reliable in that it will lead to false positives. Therefore the rewards
should not be too consequential.

To avoid massive unused data, the cards are cleared of their images every day.
And (re-)computed on demand.

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task-3495279