Changes

Read-only MessageActivity support. (#793)
* Read-only MessageActivity support.

* Removing unused code

* Requested changes by Minn

* Not failing message creation

* consistency

* requested changes

* Update
src/main/java/net/dv8tion/jda/core/entities/MessageActivity.java
 Co-Authored-By: Sanduhr32 <Maurice.Craft@t-online.de>

* Update
src/main/java/net/dv8tion/jda/core/entities/MessageActivity.java
 Co-Authored-By: Sanduhr32 <Maurice.Craft@t-online.de>
 (commit: e2b6fed)