Input groups in new Bootstrap

[Herst]

Affected checks of old Bootlint:

• E006 (.input-group contains a <select>/<textarea>) ➜ Works now with <select>/<textarea> with the .form-control/.custom-select classes respectively (https://getbootstrap.com/docs/4.1/components/input-group/#custom-select/https://getbootstrap.com/docs/4.1/components/input-group/#basic-example) ➜ Replace with new checks in future but remove for now
• E009 (Button and input sizing within .input-groups can cause issues.) ➜ Still applies? ➜ Change .input-lg into .form-control-lg
• E010 (Input groups cannot contain multiple .form-controls) ➜ Works at least with multiple <input>s except for validation (https://getbootstrap.com/docs/4.1/components/input-group/#multiple-inputs) ➜ Warning? Remove for now.
• E011 (.input-groupand.form-groupcannot be used directly on the same element) ➜ Still applies I guess ➜ Port by addingrow/.form-row`
• E012 (.input-groupand.col--` cannot be used directly on the same element) ➜ Port
• E015 (Multiple add-ons on a single side of an input group not allowed) ➜ Works now (https://getbootstrap.com/docs/4.1/components/input-group/#multiple-addons) ➜ Remove check
• E044

Note: Relevant class names changed.