Fix validation in Safari

### Problem description

Currently we rely on HTML5 validation to check the web interface forms. Safari only supports that partially, and has to be checked by hand.

On previous versions of Graylog, we were using [webshim](https://github.com/aFarkas/webshim) for supporting old browsers, but now it doesn't really fit in the new web interface, as it requires jQuery, and doesn't seem to play nice with npm and webpack.

In my opinion, the best way to solve this would be to polyfill `setCustomValidity` in Safari by displaying a bootstrap tooltip, and create our own form component that would take care of calling `checkValidity` on submit.
### Steps to reproduce the problem
1. Go to any forms not using `BootstrapFormModal` (that component already does some custom validation) in Safari, for instance the create user form
2. Submit the form without filling any fields
3. See the error message coming from the server when the request is done
### Environment
- Graylog Version: 2.0.0-alpha.6
- Browser version: Safari