As a desktop client user, I want the Shareplane invitations to be engaging for the recipients so that they are more likely to join.

In bug 1134133 we decided to implement sharing messages with a default message and no visuals to suit all share providers simply.
We now want to take this one step further and set custom messages for the main sharing providers as well as visuals when possible.

Different sharing providers available: https://activations.cdn.mozilla.net/en-US/
Based on what we see today in terms of traffic originating from social networks tells us we should focus on these top 5 providers initially: Facebook, Twitter, LinkedIn, GMail and VKontakte

Acceptance criteria:
* Facebook: custom message and custom image (defaults to "On your own Timeline")
* Twitter: custom message taking into account the fact that Twitter imposes a 140 character limit and that the Hello URL will be shortened automatically by twitter to something looking like http://t.co/vMwdg793Jf meaning there are 117 characters left to use only outside of what will be used by the URL
* LinkedIn: custom message (defaults to "Share an update")
* GMail: custom message for E-mail subject and body
* VKontakte: custom message and custom image (defaults to "Share with friends and followers")
* All Other: Default message and default picture when applicable