Add annotation count feature

[lehecht]

Display an annotation count in the annotations tab beneath the filter. If a filter is active, return an filtered annotation count and total annotation count.

Resolves #459

[mzur]

Thanks! What do you think about a style like this?

This matches the badge counts below. We only need to make sure the users understand what the count means, so I added "Total". The text uses the text-muted class so it can be distinguished from the label names.

I also thought about putting the total count at the bttom of the list which could be more intuitive. But this would hide the count if the list gets too long.

If the filtering is active, the badge could still show "yy/xx".

[mzur]

Perfect, thanks!