webstaff: Offline mode

webstaff: Offline mode

Here is implemented an offline mode interface for the web staff client.

It is made available during both network and server outages by using the
UpUp[1] service worker wrapper.

We leverage Lovefield[2] for local storage of library settings, configuration
data, offline transactions, and the standalone offline block list.

In order to make use of the offline interface, users should first log into
the web staff client and navigate to the "Search -> Search for Patrons"
interface, perform a search, select a user from the results, and open the
Patron Editor interface.  This will allow the offline interface to collect
all the relevant configuration information for the workstation.  In addition,
the offline interface available from the Circulation menu provides a "Download
block list" button when accessed while logged in.

[1]https://www.talater.com/upup/
[2]https://google.github.io/lovefield/

Signed-off-by: Mike Rylander <mrylander@gmail.com>