[FIX] web: contexts on field widgets + enforce bin_size for binary

Field widgets now have a key 'context' that let them
extend the context of the dataPoint (e.g. list).

In particular, widgets on binary fields enforce the contextual
item {bin_size: True}, so that the server gives the size of the
binary field as its value, instead of its content.

This change on binary fields reduces network load when accessing
a view with binary fields.

addons/web/static/src/js/fields/basic_fields.js

@@ -1080,6 +1080,7 @@ var AbstractFieldBinary = AbstractField.extend({
         },
         'click .o_clear_file_button': 'on_clear',
     }),
+    context: { bin_size: true },
     init: function (parent, name, record) {
         this._super.apply(this, arguments);
         this.fields = record.fields;