Pick a network device's map position, and stop hardcoding one site's label prefix

Two things a second site ran into.

The network device form asked for the map position as two raw numbers, so
placing a device meant reading coordinates off another screen and typing them
in. Machines, PCs and printers have had a "Set Location on Map" picker all
along, and the network API already accepted mapx and mapy - only the form was
missing. Same picker, same modal.

The 3D parts kiosk hardcoded 'WJ' as the prefix shown before the number box,
with a comment inviting whoever needed something else to edit the source. That
is West Jefferson's gage-lab tag format and nobody else's, so another site's
operators were told to expect letters that are not on their labels.

It is now printedparts_label_prefix, set in Settings, defaulting to EMPTY - a
site that has not set one sees no prefix rather than inheriting another site's
convention. West Jefferson sets it to WJ once. The kiosk hides the prefix
entirely when unset and falls back to no prefix if the setting cannot be read,
because a cosmetic hint must never stop a kiosk working.

Not to be confused with printedparts_code_prefix, which mints item codes like
3DP0042 and was already configurable. That is the code we generate; this is the
tag already printed on the label.

plugins/printedparts/plugin.py

@@ -126,6 +126,10 @@ class PrintedpartsPlugin(BasePlugin):
         defaults = [
             ('printedparts_code_prefix', '3DP', 'string',
              'Prefix for generated item codes'),
+            ('printedparts_label_prefix', '', 'string',
+             'Leading text on the physical gage-lab labels, shown at the kiosk '
+             'before the number box so operators type only the digits. Empty '
+             'shows no prefix. Site-specific: West Jefferson labels read WJ'),
             ('printedparts_default_threshold', '5', 'integer',
              'Default low-stock threshold for new items'),
             ('printedparts_unknown_badge', 'deny', 'string',