Save agent-id for tasks and add endpoint to get agent tasks

[anbraten]

Save which agent is running a task. This is now visible in the admin UI in the queue and in the agent details screen.

changes

• save id of agent executing a task
• add endpoint to get tasks of an agent for Autoscaler #999
• show assigned agent-id in queue
• show tasks of an agent in agent details
• (offtopic) use same colors for queue stats and icons (similar to the ones used by pipelines)
• (offtopic) use badges for queue labels & dependencies

[codecov-commenter]

Codecov Report

Patch coverage: 30.00% and project coverage change: +0.03 🎉

Comparison is base (46452fb) 39.30% compared to head (e8dfbd0) 39.33%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1631      +/-   ##
==========================================
+ Coverage   39.30%   39.33%   +0.03%     
==========================================
  Files         167      167              
  Lines       10852    10848       -4     
==========================================
+ Hits         4265     4267       +2     
+ Misses       6313     6307       -6     
  Partials      274      274              

Impacted Files	Coverage Δ
server/grpc/rpc.go	0.00% <0.00%> (ø)
server/model/step.go	43.63% <ø> (ø)
server/model/task.go	0.00% <ø> (ø)
server/queue/persistent.go	0.00% <0.00%> (ø)
server/queue/queue.go	0.00% <ø> (ø)
server/queue/fifo.go	83.33% <100.00%> (+0.13%)	⬆️

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[lafriks]

actually after looking again, maybe do not show platform if it's not specified instead of ???? It does not add any valuable information and just takes up space

[anbraten]

@lafriks Good point.