Added option to disable jQuery and heavy javascript

Shaarli uses light Javascript in its normal operation, and some jQuery
for some features (autocomplete in tags, QR-Code popup...).
jQuery can be slow on small computers. An option has been added in
configuration screen to disable javascript features which are hard on
CPU.
(Note that the Picture Wall is awfully heavy *without* jQuery.)

(Side note: A *LOT* of users want Shaarli to work without javasript at
all, if possible. That's why I try to use as few javascript as possible:
It keeps Shaarli pages fast.)