Improve player link description for screen readers

[owi92]

Previously, the alt tag was read, which doesn't convey the correct information for screen readers (somewhat implying that it links to the thumbnail, and not the video itself). Now it reads something like "Video page: {title}".

For now, this is only for the sidebox. We might want to consider doing a similar thing inside the series blocks, though there it's not as straightforward as with the sidebox link.
Also, "video" might not be the best term to use, as the events could be audio-only files. But we use "video" everywhere else, so if we use a different term, it should be updated everywhere, and I'm not sure which term would be best (probably something along the lines of "event", but that could also mean so many other things). I guess it's best to stick with "video" for now.

Fixes #661

[LukasKalbertodt]

Just some typography complaints :P

We might want to consider doing a similar thing inside the series blocks, though there it's not as straightforward as with the sidebox link.

Why is it not as easy there?
(If we don't use this translation string anywhere else, it should probably live in the "manage single video" scope instead.)

"Video" as term is fine I think.

[owi92]

We might want to consider doing a similar thing inside the series blocks, though there it's not as straightforward as with the sidebox link.

Why is it not as easy there?

Well it's not necessarily hard, but there are some additional things to consider:

• Right now, the play time is also read by screen readers. This is important information, but it's read as "4 Uhr 57" in german which does not seem optimal.
• The title beneath the thumbnail is read as well, so there is some repeated information.
• And of course the name of the uploader is also read, but without context and without pause. So this would at least need to be preceded by something like "uploaded by..." for the screen-reader

We also show the thumbnails as links on the my videos page, which is important visual information but redundant as a link and description for screen-readers, since the video titles also act as those (though they would also need to say something like "video details of...". Maybe we can do that and then skip the thumbnail for keyboard and screen-readers in this case.

(If we don't use this translation string anywhere else, it should probably live in the "manage single video" scope instead.)

I don't think that is an existent scope, is it?