Add picture_16x9 attribute for Project

This image can be use as a source for Open Graph tags, as well as for
displaying a project thumbnail with a known (or at least expected)
aspect ratio.

pillar/api/eve_settings.py

@@ -579,6 +579,8 @@ projects_schema = {
     'picture_square': _file_embedded_schema,
     # Header
     'picture_header': _file_embedded_schema,
+    # Picture 16x9 (for Open Graph)
+    'picture_16x9': _file_embedded_schema,
     'header_node': dict(
         nullable=True,
         **_node_embedded_schema

pillar/web/projects/forms.py

@@ -30,6 +30,7 @@ class ProjectForm(FlaskForm):
         ('deleted', 'Deleted')])
     picture_header = FileSelectField('Picture header', file_format='image')
     picture_square = FileSelectField('Picture square', file_format='image')
+    picture_16x9 = FileSelectField('Picture 16x9', file_format='image')
 
     def validate(self):
         rv = FlaskForm.validate(self)