Update to web-greeter master branch

[JezerM]

Why

So, this theme wasn't usable at all in the web-greeter master branch, or 3.0.0rc2 release. For some reasons I can't use the 2.2.5 release, and some lightdm methods are not compatible between these releases. Probably, these changes won't be compatible with 2.2.5, such as the use of theme_utils and greeter_config.

Changes

• Compatible with web-greeter master release
• The theme starts when lightdm is ready to use
• Some functions are now Promise based (asynchronous)
• Debug mode instantiates only when this is set in web-greeter.yml config

[eromatiya]

Thanks a lot! My laptop is broken so this is a HUGE help.

[JezerM]

Seems this PR doesn't work in web browser. I made a new PR #52. Sorry.