6pm-3: Buy and Sell working through Swagger/Backend

[gama-A]

Overview

This PR adds the functionality of buy and sell to the backend and can be used through Swagger. The proper equations for buy and sell (using the weighted scale when selling) are not implemented at the moment through this PR. (The backend tests for buy and sell are not implemented yet in this PR explaining the drop in codecov at the moment)

Issues Addressed

#14 is partially addressed in this PR

Details

The above two images showcase the buy function

The above two images showcase the sell function

[codecov]

Codecov Report

Merging #59 (65caba6) into main (b4f5ce2) will increase coverage by 18.35%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##               main      #59       +/-   ##
=============================================
+ Coverage     81.09%   99.44%   +18.35%     
=============================================
  Files            52       31       -21     
  Lines           439      181      -258     
  Branches         12        0       -12     
=============================================
- Hits            356      180      -176     
+ Misses           83        1       -82     

Impacted Files	Coverage Δ
...156/happiercows/controllers/CommonsController.java
...156/happiercows/controllers/ProfitsController.java
...happiercows/controllers/UserCommonsController.java
...cs156/happiercows/services/CurrentUserService.java
...56/happiercows/services/SystemInfoServiceImpl.java
...56/happiercows/controllers/UserInfoController.java
...56/happiercows/errors/EntityNotFoundException.java
.../cs156/happiercows/controllers/CSRFController.java
.../happiercows/controllers/SystemInfoController.java
...ucsb/cs156/happiercows/HappierCowsApplication.java
... and 11 more

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[sophiajmoore]

Made a different pull request which addresses the same changes on a different branch