refactor(api): move user logic to separate package

[jbrockopp]

Based off of #615 and #754

This change continues the refactor efforts initially introduced in the above PR.

This adds a new user package to the github.com/go-vela/server/api package.

This contains all the same handlers that existed previously but with each of them within their own file.

[codecov]

Codecov Report

Merging #809 (9bb2c47) into main (3335f6a) will increase coverage by 1.36%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #809      +/-   ##
==========================================
+ Coverage   56.83%   58.20%   +1.36%     
==========================================
  Files         243      242       -1     
  Lines       16099    15721     -378     
==========================================
  Hits         9150     9150              
+ Misses       6543     6165     -378     
  Partials      406      406              

[ecrupper]

LGTM — thank you!