Add Evidence section and fields to "add manual vulnerability" page

NOTE: This issue description assumes that #301003 (closed) is done and merged to master.

In #301003 (closed), a new "add manual vulnerability" page was created, but without the Evidence section. This issue is to add in that section:

Related designs:

Initial load showing just the "Add evidence" button	#204818[m01_Manual-result_first-run-experience.png]
All 4 field types shown at once	#204818[m03_Manual-result_vuln-info-cases.png]
States	#204818[z03_Evidence-section-details.png]

Implementation plan

• Implement the Evidence section

NOTE: This feature will be inaccessible until #301005 (closed) is complete. Because we cannot create docs that are inaccessible until the feature is done, do not add documentation for this issue.