GUI: Synoptics - add a "local" checkbox when adding PVs

As a user I would like to not have to type the full PV address, including the PV prefix, when adding a PV to a synoptic. We should handle these like we do elsewhere ie. when creating a block, where we have a "local" checkbox that prepends the PV prefix ie. `IN:ZOOM:` to the PV. This will likely require blockserver changes to make it "portable" ie. so you can copy a synoptic properly to a different instrument when copying components and the PV will be local to the copied instrument as well.

### Acceptance criteria
- [ ] GUI is modified to add a checkbox for whether or not a synoptic PV is local
- [ ] blockserver is modified to store this flag