i18n: Fix an i18n string extraction error in 02_html5_video.js

Replace a string concation to a single string to prevent the partial extraction of the string.
i18n: Make XBlock buttons & components have localizable display names

1. Current code uses type name as the display names for XBlock buttons, so the name of these five buttons can't be localized. This fix contains code that adds a 'display_name' field for the buttons, and makes their names localizable.
2. The display names of components in advanced, html and problem types can be localized now.
Fix gen_cert_report to use CourseKeys.