Quality of life changes (#968)

* + Added function to get a user's default avatar
+ Added HOST constant to Constants
+ Added assets endpoint
+ Added quality of life function to get a user's avatar or default avatar
+ Added quality of life function to get member's nickname or username

* * Fixed invocation of a getter.

* Fixed lint issue

* Made the API constant use the HOST constant for DRY-ness
Changed DOC comment to be more descriptive

* Update GuildMember.js

src/structures/User.js

@@ -104,6 +104,26 @@ class User {
     return Constants.Endpoints.avatar(this.id, this.avatar);
   }
 
+  /**
+   * A link to the user's default avatar
+   * @type {string}
+   * @readonly
+   */
+  get defaultAvatarURL() {
+    let defaultAvatars = Object.values(Constants.DefaultAvatars);
+    let defaultAvatar = this.discriminator % defaultAvatars.length;
+    return Constants.endpoints.assets(`${defaultAvatars[defaultAvatar]}.png`);
+  }
+
+  /**
+   * A link to the user's avatar if they have one. Otherwise a link to their default avatar will be returned
+   * @type {string}
+   * @readonly
+   */
+  get displayAvatarURL() {
+    return this.avatarURL || this.defaultAvatarURL;
+  }
+
   /**
    * The note that is set for the user
    * <warn>This is only available when using a user account.</warn>