[MINT-3230] About this milestone panel

[patrickseguraoddball]

MINT-3230

Description

• Added <MilestonePanel />
• Added <SolutionCard />
• Tweaked styles and functionality of solution panels
• Added link to render/add custom milestone form
• Added unit tests

How to test this change

1. Create model plan and navigate to MTO -> milestone library
2. Click on any 'About this milestone'
3. Verify figma matches
4. Verify responsiveness
5. Verify solution panels work as expected/overlay on top of milestone
6. Verify the add custom milestone works

PR Author Checklist

• I have provided a detailed description of the changes in this PR.
• I have provided clear instructions on how to test the changes in this PR.
• I have updated tests or written new tests as appropriate in this PR.
• Updated the Postman Collection if necessary.

PR Reviewer Guidelines

• It's best to pull the branch locally and test it, rather than just looking at the code online!
• When approving a PR, provide a reason why you're approving it
  • e.g. "Approving because I tested it locally and all functionality works as expected"
  • e.g. "Approving because the change is simple and matches the Figma design"
• Don't be afraid to leave comments or ask questions, especially if you don't understand why something was done! (This is often a great time to suggest code comments or documentation updates)
• Check that all code is adequately covered by tests - if it isn't feel free to suggest the addition of tests.

[garyjzhao]

things are working as described. good work!