Implement Admin Home/Export screens Re-design

[emaela]

Specs:

• Update Home/Export screen UI to match the re-design
• Add styled 'Admin' button to the home screen (only present if a participant's role is 'admin')
• Update the Export screen UI to match the re-design

Note: The Figma file and screenshot below has the bottom export section labeled as 'Round Data' and the bottom button labeled as 'Export Round CSV', but it should be 'Test Data' and 'Export Test CSV' respectively.

Additionally, if this is worked on before the test data export is implemented, then it would be acceptable for the 'Export Test CSV' button to not be functional.

Issue #183 is about adding a loading indicator to the Export screen - this is not included in this issue (this issue is only for matching the Figma design), but if the loading indicator is worked on in the same branch, please also link #183 to the PR.

Figma for Admin screens