some directory targets in wapi/wui require trailing slashes

[zooko]

Most of the URLs pointing at tahoe directories in the wapi/wui will work just as well whether there is a trailing slash after the directory cap or directory name or no trailing slash, but according to Brian on IRC:

I just tested it now, though, and without the slash broke deep-stats and deep-size and start-manifest and deep-check 'check' is ok, though