1. Add a new `view_mode` value: `embedded`.

   This value is used to determine wether the `converse-muc-embedded`
   plugin is active or not.

2. Remove the need for the #converse-embedded-chat element.

   Instead we reuse the #conversejs element and embed inside that.

3. Removed the need to manually blacklist core plugins

   This is now done automatically if `view_mode` is set to `embedded`.
   Perhaps eventually we don't blacklist at all and instead use the
   `enable` plugin method. For some that probably won't work, like
   `converse-chatview`, which we still need to be active.