add HTML5 manifest to allow offline operation/caching
manifest: add jquery library to manifest
error-box: display error message if HTTP request to server fails
offline: save/restore server_info to client's localstorage
offline: save server_info to localstorage any time it changes
offline: add/remove global CSS style when going online/offline
go_offline: if displaying an error, use the already-existing error method
offline: cache cardsets as they are loaded, for later display offline
offline: display cardsets that have been cached
offline: select correct cardset btn when showing cardset error

That way, when user sees "Cannot access this card set while offline"
they can see which cardset they had clicked on.
offline: bugfix in save_server_info

also, detect going online/offline in regular server_info refresh
offline: allow creating a cardset while offline