Team dashboards

[jpraet]

Fixes #13980

This adds the option to filter the organization "Activities" (heatmap, feed, repos), "Issues", "Pull Requests" and "Milestones" views by team.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@25f8970). Click here to learn what that means.
The diff coverage is 41.53%.

@@            Coverage Diff            @@
##             master   #14159   +/-   ##
=========================================
  Coverage          ?   42.03%           
=========================================
  Files             ?      733           
  Lines             ?    78697           
  Branches          ?        0           
=========================================
  Hits              ?    33080           
  Misses            ?    40184           
  Partials          ?     5433           

Impacted Files	Coverage Δ
models/action.go	50.93% <0.00%> (ø)
models/repo_list.go	78.76% <0.00%> (ø)
routers/user/home.go	60.27% <32.25%> (ø)
models/org.go	72.51% <43.75%> (ø)
models/user_heatmap.go	81.57% <100.00%> (ø)
routers/api/v1/repo/repo.go	61.89% <100.00%> (ø)
routers/routes/macaron.go	93.17% <100.00%> (ø)

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