display uncertainty of authorship on works (preview and single view)

[peterstadler]

See A021597 where the encoding of both composers is flagged with cert="medium"

[riedde]

I have a comparable problem conerning named persons. A solution could be to render the certainty in the popover title (app:preview-title), but the information of the certainty is not available there. The title is get by a call of the referenced object, so the information about what is encoded beside is not available. I tried to add this to the html and ended up with having an attribute in the link to the popover (<a class="preview persons A000438D" href="/de/A000438D.html" cert="low">Peter H</a> taken from HenDi: A04028E3). This could be a solution, if we read and process this information. Sadly I wasn't able to do this.