Fix admin list user/org API

[jolheiser]

Resolves #6126

Previously, a page size of -1 would cause a panic when making the slice. users = make([]*User, 0, opts.PageSize)
Now, in the user search API the page size will be checked for <=0 in order to enforce page sizes.
Admins, however, will be able to use the -1 page size to get back all users/orgs

[codecov-io]

Codecov Report

Merging #6143 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #6143      +/-   ##
==========================================
- Coverage   38.86%   38.84%   -0.02%     
==========================================
  Files         354      354              
  Lines       50112    50115       +3     
==========================================
- Hits        19474    19468       -6     
- Misses      27820    27829       +9     
  Partials     2818     2818

Impacted Files	Coverage Δ
models/user.go	49.72% <0%> (-0.14%)	⬇️
routers/api/v1/user/user.go	23.52% <0%> (ø)	⬆️
models/unit.go	0% <0%> (-14.29%)	⬇️
routers/repo/view.go	44.54% <0%> (-1.15%)	⬇️

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