fix(business/button): add icon content child to component and template

[efux]

I added to necessary instructions directly to the business button. This hopefully displays the icons again. If I have time, I will someday refactor the stylings, so there is another template, but for the moment, I would just like to get it to work. So a bugfix release asap would be appreciated, otherwise I would have to rollback to an older version.

In my tests the directive

<button sbbButton mode="icon">
  <sbb-icon-arrow-right *sbbIcon></sbb-icon-arrow-right>
</button>

still did not work, but I hope this is due to my test setup. However,

<ng-template #icon>
  <sbb-icon-arrow-right></sbb-icon-arrow-right>
</ng-template>
<button sbbButton mode="icon" [icon]="icon">

works fine.