Improve "social network" tab's form field's labels in petition edit page

[fallen]

For now the form fields are:

• Twitter description
• Twitter image
• Org Twitter handle

The first two should be renamed so that it more clearly states what it is for.
Also, for the first two, "Twitter" should not be part of the name since it will work for any other social network parsing the "Open graph" tags.

Issue is that the current form field labels are definitely not helping to understand what the data will be used for