[Bootstrap 4] Add class `show` to alerts

[YiiRocks]

What steps will reproduce the problem?

Use yii2-usario with bootstrap 4

What is the expected result?

Alerts to show

What do you get instead?

Alerts are invisible

I know you don't support bootstrap 4 at this point, but would it be able to add the class show to all alert windows for now, so they at least show up? All I get now during registration and stuff is white pages. And yes, I know I could override those pages ;)

[tonydspaniard]

@maxxer should we create a version for B4? What do you think?

I wish I could do it CSS framework agnostic though

[maxxer]

I was about to, I also had it locally... but haven't had time to test and make few enhancements.

Also, from what I've read Yii2.1 is now Yii3 (because they're now following Semver) so I didn't check all the stuff.

I preferred to merge GDPR