Fix clickable links in PDF viewer

[jdmcleod]

Summary

• add a regression test that verifies PDF link annotations render as clickable anchors
• render annotation links in rm-pdf-page using PDF annotation data and viewport coordinates
• layer annotation links above text/canvas so links are actually clickable

Verification

• yarn test --run test/components/pdf-viewer.test.js (red before fix, green after)
• yarn test --run
• yarn build