Show "delete branch" button on closed pull requests (#6570)

[saitho]

Resolves #6570

Note that these changes introduce a new behaviour:
Closed pull requests whose pull branch was deleted can't be reopened by the user anymore.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@b1bb700). Click here to learn what that means.
The diff coverage is 66.66%.

@@            Coverage Diff            @@
##             master    #6601   +/-   ##
=========================================
  Coverage          ?   40.78%           
=========================================
  Files             ?      421           
  Lines             ?    57571           
  Branches          ?        0           
=========================================
  Hits              ?    23480           
  Misses            ?    30965           
  Partials          ?     3126

Impacted Files	Coverage Δ
routers/repo/issue.go	37.15% <100%> (ø)
routers/repo/pull.go	36.67% <50%> (ø)

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[saitho]

What needs to be done in order to merge this? :)

[lunny]

@saitho need two maintainers' approvals.

[zeripath]

One small nit - I think it makes the comment clearer.