feat(GuildEmoji): add deletable getter (#2535)

* feat(GuildEmoji#deletable)

* Update: Discord API returns 50001

When you try to get or delete a managed emoji from the API, you get a 50001 (Missing Access) error code.

src/structures/GuildEmoji.js

@@ -1,4 +1,5 @@
 const GuildEmojiRoleStore = require('../stores/GuildEmojiRoleStore');
+const Permissions = require('../util/Permissions');
 const Snowflake = require('../util/Snowflake');
 const Emoji = require('./Emoji');
 
@@ -44,9 +45,20 @@ class GuildEmoji extends Emoji {
     return clone;
   }
 
+  /**
+   * Whether the emoji is deletable by the client user
+   * @type {boolean}
+   * @readonly
+   */
+  get deletable() {
+    return !this.managed &&
+      this.channel.permissionsFor(this.client.user).has(Permissions.FLAGS.MANAGE_EMOJIS);
+  }
+
   /**
    * A collection of roles this emoji is active for (empty if all), mapped by role ID
    * @type {GuildEmojiRoleStore<Snowflake, Role>}
+   * @readonly
    */
   get roles() {
     return new GuildEmojiRoleStore(this);