Allow sending HTML newsletters from an external source

[fishilico]

The School would like to send its newsletter through Polytechnique.org. As they already provide a web page with the content of this newsletter (e.g. http://eye.sbc41.com/w/17848/W-gtTpfEDEuXmTqMPuw21Q/ylkkbj_-_Uqd-Kj_rbq5Mw/5uLqso1qIkmmaxUi1xw4TA), the simpler way seems to consist in grabbing such a page, adding an unsubscription link to the end, creating an email and sending it like any newsletter which are already handled.

As of today, I have:

• modified the database to record what needs to be recorded about a web-based newsletter.
• implemented the UI integration to create a newsletter out of a web page.

Works still need to be done:

• to transform the web page to an email which will be sent (a library may exist which does that, in PHP or in any other programming language).
• to add a footer with the subscription link to the end of the email
• to actually send the email with the existing infrastructure (which should be easy).