PLT-3383: image proxy support (#7991)

* image proxy support

* go vet fix, remove mistakenly added coverage file

* fix test compile error

* add validation to config settings and documentation to model functions

* add message_source field to post

i18n/en.json

@@ -4814,6 +4814,14 @@
     "id": "model.config.is_valid.listen_address.app_error",
     "translation": "Invalid listen address for service settings Must be set."
   },
+  {
+    "id": "model.config.is_valid.image_proxy_type.app_error",
+    "translation": "Invalid image proxy type for service settings."
+  },
+  {
+    "id": "model.config.is_valid.atmos_camo_image_proxy_options.app_error",
+    "translation": "Invalid atmos/camo image proxy options for service settings. Must be set to your shared key."
+  },
   {
     "id": "model.config.is_valid.localization.available_locales.app_error",
     "translation": "Available Languages must contain Default Client Language"