Add support for search by uid

[zeripath]

At the moment there is no way to search for users by their ID using the API. Gitlab does this by overloading the /users/{username} API routes to allow searching with user ID. Rather than go that route this pull request adds a uid parameter to the /users/search API and allows searching by user ID.

[codecov-io]

Codecov Report

Merging #4876 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #4876      +/-   ##
==========================================
- Coverage    37.4%   37.38%   -0.02%     
==========================================
  Files         306      306              
  Lines       45389    45397       +8     
==========================================
- Hits        16978    16972       -6     
- Misses      25956    25967      +11     
- Partials     2455     2458       +3

Impacted Files	Coverage Δ
models/user.go	43.67% <0%> (-0.13%)	⬇️
routers/api/v1/user/user.go	9.8% <0%> (-0.51%)	⬇️
models/repo_indexer.go	48.3% <0%> (-3.82%)	⬇️
modules/process/manager.go	81.15% <0%> (+4.34%)	⬆️

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