Remove hardcoded "edX" mentions in the strings

[OmarIthawi]

There's a couple of "edX" strings that needs to be made dynamic into {platform_name}:

$ make extract_translations
$ cat src/i18n/transifex_input.json | grep -i edx
  "footer.socialLinks.srText.facebook": "Like edX on Facebook",
  "footer.socialLinks.srText.twitter": "Follow edX on Twitter",
  "footer.socialLinks.srText.youtube": "Subscribe to the edX YouTube channel",
  "footer.socialLinks.srText.linkedin": "Follow edX on LinkedIn",
  "footer.socialLinks.srText.reddit": "Subscribe to the edX subreddit",
  "footer.edxLinks.business": "edX for Business",
  "footer.mobileApp.apple": "Download the edX mobile app from the Apple App Store",
  "footer.mobileApp.google": "Download the edX mobile app from Google Play",
  "footer.logo.ariaLabel": "edX Home",