Allow users to edit the Advanced JSON Message Template

[lytzV]

1. Are you using Helium hosted Console or running your own open source Console? Hosted

2. If you are on Helium hosted Console, which url are you currently on? This issue is observed across staging, prod and dedicated console)

Please describe your issue: Users cannot edit the example JSON message and can only use the Default JSON, Browan JSON or Cayenne JSON to test their mustache parsing. This is highly inconvenient considering users might want to test with the JSON message that they actually received instead of an arbitrary example. (e.g. they may have a very different decoded payload structure)

Users should be able to test their mustache parsing with their own JSON message examples, which can be obtained from their event log or the console debug tool.