Feat: routes for viewing invoice

[lohanidamodar]

What does this PR do?

• Add route to view PDF invoice
• Add route to download PDF invoice
• Each route validates that invoice exists and then redirects to the REST endpoint

Test Plan

• Manually tested it redirects to correct endpoint upon success
• Manually tested it throws error if invoice doesn't exist

Related PRs and Issues

(If this PR is related to any other PR or resolves any issue or related to any issue link all related PR and issues here.)

Have you read the Contributing Guidelines on issues?

YES

[ItzNotABug]

we already handle navigating to a different URL if the visitor doesn't have a session, right?

[lohanidamodar]

we already handle navigating to a different URL if the visitor doesn't have a session, right?

Yes we do.