Video Upload UI Edits

Now that we are doing video upload through Studio, it would be nice to just refer to that: "If you have uploaded videos through Studio..." However, that language doesn't include our users who already have a bunch of video assets that have been processed through the old FTP-based workflow. One option would be to use the PLATFORM_NAME setting to customize the string. We definitely shouldn't refer to Open edX, because most users will only be familiar with the name of the site (e.g. XuetangX, Edraak) but not the underlying technology.