Correct icon classname for user image fallback

Without the "icon" class name, the fallback icon is not displayed properly.
Fix broken combobox pagination
Center pagination buttons regardless of listwidth
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.
Cleanup the resource getnodes processor

Prior to this cleanup, due to an issue with ltrim(), it was impossible for custom resources to use a custom icon. After this cleanup that ability is restored through a system setting (that 3PCs need to provide) of mgr_tree_icon_<CLASS_KEY> where <CLASS_KEY> is a lowercase representation of the class key with, if applicable, the "mod" prefix removed.

On top of that, this clean up also resolves duplicate classes being assigned to the icon. It also improves the readability and fixes some weird and/or duplicated logic.
Update phpThumb to v1.7.13
Enable usernav menus for mobile devices
Fix long category name overflow in vertical tabs
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.
Fix Files Tree Hover-Preview Thumbnail border not expanding to correct width
Fix excessive space in package update window
Fix labels / tvs in custom FC tabs
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
Fix pagination on Topic combobox

Addresses issue #11713

Merge remote-tracking branch 'origin/pr/11720'

* origin/pr/11720:
  Center pagination buttons regardless of listwidth
  Fix broken combobox pagination
Cleanup and enhance Resource getNodes processor

Merge remote-tracking branch 'origin/pr/11736'

* origin/pr/11736:
  Cleanup the resource getnodes processor
Update phpThumb release to v1.7.13

Merge remote-tracking branch 'origin/pr/11742'

* origin/pr/11742:
  Update phpThumb to v1.7.13
Enable usernav menus for mobile devices

Should address #11600, #11704 and most of #11706 (menus aren't easily closed on mobile per :hover)

Merge remote-tracking branch 'origin/pr/11743'

* origin/pr/11743:
  Enable usernav menus for mobile devices
Fix long category name overflow in vertical tabs

Addresses #11728

Merge remote-tracking branch 'origin/pr/11750'

* origin/pr/11750:
  Fix long category name overflow in vertical tabs
Fix hover-preview thumbnail border width in Files tree

Merge remote-tracking branch 'origin/pr/11752'

* origin/pr/11752:
  Fix Files Tree Hover-Preview Thumbnail border not expanding to correct width
Fix excessive space in package update window

Merge remote-tracking branch 'origin/pr/11757'

* origin/pr/11757:
  Fix excessive space in package update window