Drop user shutter requests based on LSS shutter enable state

[klauer]

Per Tyler, as recommended by the laser group:

The BTPS should drop user requests based on the LSS shutter enable state. E.g. if the shutter enable state goes low (disallowed) and the user is requesting the shutter to open, the user request should be forced low until the LSS shutter enable is high and the user once again asks the shutter to open. Basically the LSS is worried about the shutter being opened quickly after the LSS loses permission. They want to force the user to request the open state again.

[klauer]

We have open status, close status (i.e., limit switch inputs) and also permission status. We should:

• Re-add the permission status input for ShutterSafety: https://github.com/pcdshub/lcls-plc-las-bts/commit/b042cccbcd54306e4ee149bb8c8f876c80fe2986
• Link it to ST_ShutterStatus.bLssStatus
• And then force the user request low when the permission status transitions high-low.
• Override mode can probably stay high (not included in the above request) - but @slactjohnson may have a different thought on that one.