Fixed too generic checkbox and link selectors

It turns out that the sharing dropdown is embedded into the table row
for some reason.

This fix makes use of a more specific selector for the checkboxes and
file name links to make sure it doesn't spill into any other potential
elements like the sharing dialog.

CC @DeepDiver1975 as discussed
Please review @schiesbn @jancborchardt @MorrisJobke 

Before the fix: the checkboxes from the sharing dropdown cause the whole file to be selected
After the fix: correct isolation between file list checkbox and share dropdown checkbox