Synoptic: Labels should track block names

As an IS on IRIS I would like the labels in my synoptic to match my block names and keep track of them. E.g. the following use case:
* I create a config with a block called `sample_temp` pointing at `EUROTHERM_01:TEMP1` (config 1)
* I create a synoptic with a PV value based on that block
* I create a config with a block called `cryostat_temp` pointing at `EUROTHERM_01:TEMP1` (config 2)
* I switch from config 1 to config 2

Under current behaviour my synoptic will still have a PV on it labelled `sample_temp` and pointing at `EUROTHERM_01:TEMP1`. Instead I would like the synoptic to have a PV on it labelled `cryostat_temp` and pointing at `EUROTHERM_01:TEMP1`.

Note #2841 is effectively the opposite of this in that it will track the linked PV rather than the block label. Make sure these two don't clash

## Acceptance Criteria

* Blocks added to synoptics can track block labels
* There is a system test for this
* Default behaviour is unchanged