env.sh: Use perl to compute GOPATH on systems with a non-GNU readlink

This includes for example OS X systems.
monitor: Ignore events that only touch file attributes

On OS X (at least), reading a picture to thumbnail it generates a new
fsnotify event since the atime is updated, making the thumbnailer read
the file again to update the thumbnail. Ignoring file attribute changes
allows breaking that loop.
thumbnailer: Don't thumbnail files which are not in albums
ui: Add an oops screen for showing general errors
ui: Oops when not being able to load some JSON or an image
ui: Escape # in picture URLs
ui: Factor URL computation in a common method

I didn't feel too good with those String.replace all over the place...