Rework verbose mode

[agateau-gg]

What has been done

This PR reworks ggshield verbose mode. It does a few different things:

• Make --debug automatically enable verbose mode, so there is no need to add both -v and --debug.
• Unify the way we output verbose messages by using ui.display_verbose() instead of

  if verbose:
       click.echo(...)
  

• As a consequence of the previous point: remove lots of verbose arguments from functions. They are no longer necessary because the UI now knows its verbosity level.

Review

This PR is quite large, because verbose is everywhere. As usual, best reviewed commit by commit.

Validation

• Run a ggshield command with --debug: notice the verbose output is there