make nuxeo-document-create-button accessible

nuxeo-document-create-button needs an aria-label to make it accessible.

From axeDevTools:

Issue Description
Ensures every ARIA button, link and menuitem has an accessible name

Element location
nuxeo-app nuxeo-document-create-button #createBtn
Element source
<paper-fab id="createBtn" noink="" icon="nuxeo:add" aria-labelledby="createBtnTooltip" role="button" tabindex="0" animated="" elevation="1" aria-disabled="false"></paper-fab>
To solve this violation, you need to...
Fix at least one (1) of these issues:
aria-label attribute does not exist or is empty

aria-labelledby attribute does not exist, references elements that do not exist or references elements that are empty

Element has no title attribute

Element does not have text that is visible to screen readers