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.
