Change default names for various LMS components

Embedded discussion component defaults to "Discussion"
Blank HTML page defaults to "Text"
Video component defaults to "Video"

These default names show up in tooltips.

common/lib/xmodule/xmodule/html_module.py

@@ -25,7 +25,7 @@ class HtmlFields(object):
         scope=Scope.settings,
         # it'd be nice to have a useful default but it screws up other things; so,
         # use display_name_with_default for those
-        default="Blank HTML Page"
+        default="Text"
     )
     data = String(help="Html contents to display for this module", default=u"", scope=Scope.content)
     source_code = String(help="Source code for LaTeX documents. This feature is not well-supported.", scope=Scope.settings)