Provide default backend replacing example backend

Problem/Motivation

As example backend will gather more features:

1. it will become not a good example, but a bloated example
2. it will become not an example, but a solid backend
3. users will use it by default instead of customizing it

therefore its good to replace "example backend" with "default backend" and move to the main module.

Steps to reproduce

Proposed resolution

1. Copy "example backend" to the main module and rename it to "default backend".
2. Remove most code from "example backend" and extend "default backend", and add some comments for developers how to make backends.
3. Create ExampleBackend no. 2 demonstrading how to extend base class.
4. Edit docs making "example backend" optional, because users won't need to enable in order to use the chat

Remaining tasks

User interface changes

API changes

Data model changes