Refactor remove-user command and output failed sites

[ahegyes]

Refactors the remove-user command to bring it in line with the commands added in the last 9 months or so thus removing the dependency on the deprecated API_Helper and making use of more reusable functions.

At the same time, provides a warning for when a WPCOM site could not be queried as suggested by #162.

[nickpagz]

@ahegyes This looks and works great, though I did come across a warning, and there was one slightly confusing bit in the output, not a big deal, but bringing it up anyways.

First, I tested with --list, then without --list and aborting, and finally without --list and confirming removal.

The confusing part:
I added my personal email as a collaborator on tumblr-swag-store-development.mystagingwebsite.com via Pressable without WP Access. I was already a "customer" on the site, but the command reports me as a "collaborator". It was confusing since the site shows up twice, once as a pressable site ID, and then again as a wpcom site ID. I understand why it shows up twice, it just took me a couple minutes to understand why.
Now, this is probably not a big deal for our 51 sites, but in a perfect world we could check for type of access before removing the user, and only remove users with a certain level of access. Having said, we probably couldn't check for custom roles or permissions and it's probably better to just nuke the user anyways (and add them back later if needed).

Overall though, it works perfectly as intended.

The PHP warning:
This showed up on the confirmation run:
PHP Warning: Undefined property: stdClass::$email in /Users/nickp/team51-cli/src/commands/remove-user.php on line 128
I think line 128 should be $this->user, or, add 'email' in the returned array on line 215. I didn't fully debug though, so not 100% sure on what's included in that object.

A compiled screenshot of the output: