lli - Add popup modal when clicking buy/sell cows button

[lukewli]

Overview

In this PR, I've implemented a popup modal when users are clicking on buying and selling cows that will let them choose how many cows they want to buy or sell in one go instead of having to push the buy/sell button multiple times.

Dokku Deployment: https://5pm-1-happycows.dokku-01.cs.ucsb.edu/

Screenshots (Optional)

Before

After

Validation

1. Download this branch.
2. Run the project.
3. Create/join a common.
4. Test the popup modal when buying and selling cows.

Tests

• [x] Backend Unit tests (mvn test) pass
• [x] Backend Test coverage (mvn test jacoco:report) 100%
• [x] Backend Mutation tests (mvn test pitest:mutationCoverage) 100%
• [x] Frontend Unit tests (npm test) pass
• [x] Frontend Test coverage (npm run coverage) 100%
• [x] Frontend Mutation tests (npx stryker run) 100%
• [x] Frontend Linting (npx eslint --fix src)

Linked Issues

Closes #8

[haroldmo]

Tested locally and the the buy and sell features work as intended. All tests passed locally as well.