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

config/default.json

@@ -56,7 +56,9 @@
         "EnablePreviewFeatures": true,
         "CloseUnusedDirectMessages": false,
         "EnableTutorial": true,
-        "ExperimentalEnableDefaultChannelLeaveJoinMessages": true
+        "ExperimentalEnableDefaultChannelLeaveJoinMessages": true,
+        "ImageProxyType": "",
+        "ImageProxyURL": ""
     },
     "TeamSettings": {
         "SiteName": "Mattermost",