Removed `_converse.chatboxviews.showChat` and trying to simplify how
chats are created and when they're shown.
Prompted by the work to split the MUC views into a separate plugin
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.
- Use `enabled` method for plugins
- Rename `converse-inverse.js` to `converse-fullscreen.js` and make it a
core plugin.
- We're no longer making separate mobile and inverse builds
No longer a need to whitelist/blacklist plugins for inverse to work.
So that when a room is closed, a previously obscured chat doesn't become
visible again. Allowing this would force us to know which chat is visible, so
that we can falsify it's `hidden` attribute.
