Unlock layer when locked by a runner that does not exist anymore

[LucasMrqes]

For unknown reasons, it happens that Burrito locks a layer and never unlocks it (probably because a pod was killed due to a spot instance reclaim). That causes all the next runs to fail, as the layer is locked.
To prevent this behavior, we could implement a way to unlock the layer before starting a plan / apply operation only if the following conditions are met:

• The layer was locked by a Burrito run
• The pod that locked the layer is not running anymore

[LucasMrqes]

Furthermore, show in the UI when a layer is locked, that could be useful if somebody (or an automated process) left a lock on a layer