Add HostCertificate lines to sshd_config

[aes]

Afaik, adding these lines does nothing unless the file(s) are present. Having them in let's admins supply certs instead of relying on TOFU.

[codecov-io]

Codecov Report

Merging #13143 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #13143      +/-   ##
==========================================
- Coverage   42.13%   42.11%   -0.02%     
==========================================
  Files         678      678              
  Lines       75041    75041              
==========================================
- Hits        31617    31603      -14     
- Misses      38275    38292      +17     
+ Partials     5149     5146       -3     

Impacted Files	Coverage Δ
models/repo_mirror.go	2.38% <0.00%> (-11.91%)	⬇️
modules/cron/tasks_basic.go	87.35% <0.00%> (-3.45%)	⬇️
models/unit.go	46.57% <0.00%> (-2.74%)	⬇️
modules/log/event.go	59.90% <0.00%> (-1.89%)	⬇️
modules/git/blame.go	75.43% <0.00%> (-1.76%)	⬇️
services/mirror/mirror.go	18.51% <0.00%> (-1.35%)	⬇️
routers/repo/view.go	37.47% <0.00%> (-0.65%)	⬇️
models/issue_comment.go	52.93% <0.00%> (-0.16%)	⬇️
models/error.go	34.39% <0.00%> (ø)
modules/util/timer.go	85.71% <0.00%> (ø)
... and 4 more

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[zeripath]

This could go into 1.13 if it is approved quickly enough.