Expose user avatar URL field in the UI (#27)

* wip

* some fixes

* update readme

* update readme

* Add option to change/erase any user's avatar

* Fix README

* Remove mutationMode from Edit

* remove log

* update readme

src/synapse/authProvider.test.ts

@@ -101,7 +101,7 @@ describe("authProvider", () => {
     });
 
     it("should reject if error.status is 401", async () => {
-      await expect(authProvider.checkError({ status: 401 })).rejects.toBeUndefined();
+      await expect(authProvider.checkError(new HttpError("test-error", 401, {errcode: "test-errcode", error: "test-error"}))).rejects.toBeDefined();
     });
 
     it("should reject if error.status is 403", async () => {