Fix missing stylesheets on installation page

[shilch]

When running gitea for the first time, the stylesheets for the
installation page are broken since the middleware that statically serves
stylesheets does not get executed by chi. This is because if no handlers
are registered in chi, it will drop all middleware.

This commit introduces a "dummy" handler to deal with that quirk.

Closes #13725

Thanks: Lunny Xiao xiaolunwen@gmail.com for finding the quirk

[codecov-io]

Codecov Report

Merging #13736 (6ba56ce) into master (1e5247d) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master   #13736      +/-   ##
==========================================
- Coverage   42.21%   42.19%   -0.03%     
==========================================
  Files         697      697              
  Lines       76845    76848       +3     
==========================================
- Hits        32439    32423      -16     
- Misses      39054    39072      +18     
- Partials     5352     5353       +1     

Impacted Files	Coverage Δ
routers/routes/chi.go	35.18% <0.00%> (-0.67%)	⬇️
services/pull/check.go	48.90% <0.00%> (-3.65%)	⬇️
services/pull/temp_repo.go	26.59% <0.00%> (-3.20%)	⬇️
modules/process/manager.go	72.50% <0.00%> (-2.50%)	⬇️
services/pull/patch.go	53.97% <0.00%> (-1.71%)	⬇️
modules/queue/workerpool.go	58.77% <0.00%> (-1.23%)	⬇️
models/error.go	36.25% <0.00%> (-0.50%)	⬇️
modules/indexer/stats/db.go	52.17% <0.00%> (ø)
modules/git/repo.go	45.68% <0.00%> (+0.50%)	⬆️
modules/git/utils.go	77.04% <0.00%> (+3.27%)	⬆️

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