Correct icon classname for user image fallback

Without the "icon" class name, the fallback icon is not displayed properly.
Add check for allow_url_fopen then CURL fallback

Add a check to see if allow_url_fopen is enabled then fallback to CURL if available.
Simplify Gravatar retrieval and fallback

Utilize Gravatar's built in fallback image so that we don't need to check for an existing profile image on every manager pageload.
Simplify Gravatar profile image fallback

Merge remote-tracking branch 'origin/pr/11716'

* origin/pr/11716:
  Simplify Gravatar retrieval and fallback
  Add check for allow_url_fopen then CURL fallback
  Correct icon classname for user image fallback