SHL-140: Fix 'help' command completion

Don't mind the formatting mess (created SHL-141 for that btw), what this does is:

- use @Component for converters
- fix the AvailableCommandsConverter

I've already closed (as 'duplicate') SHL-140, because it is a sub-part of another issue (feel free to proceed otherwise. Can close 90 for example, as 90 = 140 + 89). The contents of this PR needs to be merged nevertheless