Groups for Apps

**Is your feature request related to a problem? Please describe.**
No

**Describe the solution you'd like**
The ability to group apps together and view all the messages by group. Currently you can view messages from a particulate app or you can view all messages, it would be nice to selectively view all messages from a groups of apps.

**Describe alternatives you've considered**
I have considered deploying separate Gotify instances for different groups of apps but it's also desirable to be able to view all messages.

**Additional context**