Visual indicator for stalled workflow in sidebar

[MetRonnie]

Problem

One of these workflows is stalled while the other one simply has a deprecation warning

(Workflows can stall even if there are no failed tasks, due to e.g. missing custom outputs)

Proposed Solution

Add a workflow icon for stalled workflows, possibly in a warning colour. Even if we reduce the noisiness of the warning triangle, I think that stalled is important enough to have its own separate indicator.

PoC:

Additional context

The idea of having a separate status for stalled was rejected in https://github.com/cylc/cylc-flow/issues/5256, but we may want to revisit?

[dpmatthews]

I think we should indicate if the flow is stalled or has incomplete tasks (which implies the workflow will eventually stall) - in both cases intervention is required in order for the workflow to continue / complete