Use custom type and constants to hold available order by options

[Morlinest]

Extracted from #2371. Replace strings for constants.

[codecov-io]

Codecov Report

Merging #2572 into master will increase coverage by <.01%.
The diff coverage is 23.52%.

@@            Coverage Diff             @@
##           master    #2572      +/-   ##
==========================================
+ Coverage   27.32%   27.33%   +<.01%     
==========================================
  Files          86       86              
  Lines       17135    17137       +2     
==========================================
+ Hits         4682     4684       +2     
  Misses      11775    11775              
  Partials      678      678

Impacted Files	Coverage Δ
routers/user/profile.go	0% <0%> (ø)	⬆️
models/repo_list.go	24.72% <57.14%> (+0.83%)	⬆️

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[lafriks]

LGTM

[daviian]

LGTM

[lafriks]

Actually just reviewed it again and does that function is needed? Why not to use string(x) where needed?

[Morlinest]

Because you don't need to care what type SearchOrderBy is (try to think about it like you don't know that it's only string) and this way it implements Stringer interface. string(x) will not work without other changes if you change SearchOrderBy to int for example. Changing constants should not affect other code.