Use the new sbbslist_lib methods for encoding/decoding (decompressing)

and displaying/showing login previews. This saves over 50% of the storage
for login previews and will reduce the redundant chatter on the message
bases where these previews may be exported.
Previous versions of sbbslist.js will not be able to decode LZ-compressed
previews, but should do so gracefully (no error, just no preview).
Fixed the "update -preview" option (wasn't capturing previews).
This file is now dependent on load/lz-string.js and hexdump_lib.js.