workflows: indicate held and retrying tasks

[oliver-sanders]

It would be useful for monitoring if the workflows view could provide some sort of indicator of held and retrying tasks.

[MetRonnie]

It was suggested to just use the modifier icons rather than adding more coloured badges with numbers in them.

Quick mock-up (based on top of #2331):

[oliver-sanders]

Note: We reckon that it is more important to indicate that there are tasks in these states than to indicate the exact number.

Space is at a premium here!

[oliver-sanders]

Blocked by cylc/cylc-flow#7071

[hjoliver]

Agreed, the mockup looks great.