Add file-upload component

[azerella]

With the recent release of the form component and the draft by @toxamiz in #680 we have an opportunity to publish a simplified version of the file-upload component.

This PR only features the semantic basic functionality. We can look at another advanced release with the work from @toxamiz but that should take more time

Preview:

Preview HTML:

<div class="au-file-upload au-form-group">
    <label class="au-label" for="file-upload">Upload a file</label>
    <input class="au-file-input" type="file" id="file-upload"></input>
</div>

Todo:

• Site documentation
• Component image
• Community discussion

[azerella]

Blocked until #727 is merged