Cached avatar support (from Rust) (#633)

* loading a cached avatar at first, and then actually doing the request so that the avatar can still be displayed without connection

* improved the code

* fix + changelog

* policy on how to fetch the avatar url added

* code improvements

* addressed al PR comments

* improved the error handling

* improved the async task code

* fixed a typo

* fixed another typo

Dangerfile.swift

@@ -49,7 +49,8 @@ let allowList = ["stefanceriu",
                  "manuroe",
                  "gileluard",
                  "phlniji",
-                 "aringenbach"]
+                 "aringenbach",
+                 "Velin92"]
 
 let requiresSignOff = !allowList.contains(where: {
     $0.caseInsensitiveCompare(danger.github.pullRequest.user.login) == .orderedSame