Introduce Callout

[TheSyscall]

Implement a callout box element that can be used to convey important information to the user.
This element is designed to be used above certain form elements, over the whole form or page.

It seems like the use of callout could conflict with the existing FormDescription and FormNotification decorators, at least within IW2 forms.
Callouts are not meant as a direct replacement for these decorators but as a more universal ipl widget that can be used even outside of forms to convey essential information to the user.

Merging these two concepts by extending the form-notification would be a viable alternative to this PR.

Callout types:

Callout full width:

Callout with optional title:

Callout above a form element:

closes #349

[flourish86]

I like the design, especially the color matching border. The spacing of text and headline are well done.

We should check, though, if there are elements, that have the same or a similar purpose and make sure that the design is consistent.

Please make also sure, that the in this case the line breaks after max. 80 characters to ensure readability.
This is usually done by setting a max width of ~50em to the element containing the paragraph.

@TheSyscall Would be nice, if you could post a screenshot of the element positioned on top of the page.

[TheSyscall]

On the top of a configuration form without any limit. (limited only by the content)

Limited to 80ch/50em.

Setting the limit like that breaks the use over form elements.
Note the large blank space on the right side of the callout.

[flourish86]

lgtm

[Al2Klimov]

Tested together with Icinga/icingaweb2#5477 (review).