Add button for toggling status badge visibility on PVTableModels

[shilorigins]

Showing status badges for every cell containing EPICS data (as introduced in SWAPPS-420) may appear messy, especially when lots of badges are active at once

Add a button to the snapshot page that toggles the visibility of such badges. The button may toggle visibility for all snapshot tables, so the toggle may be stored as a global variable in pv_table.py, or as an attribute of each table model.

Visibility may be active or inactive on launch, I'm undecided.

SWAPPS-468