Use ServerError provided by Context

[lunny]

ctx.InternalServerError(err) could not be used with context.Context because it's a method of macaron.Context or context.APIContext.

Bugs introduced in #14243, #10967, #9784, #8346

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@edbc5c8). Click here to learn what that means.
The diff coverage is 0.00%.

@@            Coverage Diff            @@
##             master   #14333   +/-   ##
=========================================
  Coverage          ?   41.86%           
=========================================
  Files             ?      744           
  Lines             ?    79638           
  Branches          ?        0           
=========================================
  Hits              ?    33342           
  Misses            ?    40811           
  Partials          ?     5485           

Impacted Files	Coverage Δ
routers/admin/users.go	38.84% <0.00%> (ø)
routers/repo/issue.go	38.17% <0.00%> (ø)
routers/repo/projects.go	13.42% <0.00%> (ø)
routers/repo/pull.go	31.79% <0.00%> (ø)

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