Cleanup: renaming "frame" to "character"

The "frame" variable actually contains a string pointing to the character selected.
It has nothing to do with a frame which is usually a particular image in an animation.

I'm renaming the variable accross the application to avoid confusion.

back/tests/MessageTest.ts

@@ -8,7 +8,7 @@ describe("Message Model", () => {
         expect(messageObject.userId).toBe("test1");
         expect(messageObject.roomId).toBe("test2");
         expect(messageObject.name).toBe("foo");
-        expect(messageObject.frame).toBe("user");
+        expect(messageObject.character).toBe("user");
     })
 
     it("should expose a toJson method", () => {
@@ -37,4 +37,4 @@ describe("Message Model", () => {
             let messageObject = new Message(message);
         }).toThrow(new Error("userId or roomId cannot be null"));
     });
-})
+})