Add a temporary commonPage.ts to eventually replace htmlPage.ts

Tucker McKnight <tmcknight@instructure.com> | Wed Feb 11 2026

Add a temporary commonPage.ts to eventually replace htmlPage.ts

This new commonPage.ts should include everything that overlaps between
what was previously in js_templates/index.ts and
js_templates/common/htmlPage.ts.

The goal is to have *one* htmlPage.ts that is used on all pages.

For now, js_templates/index.ts uses this new commonPage.ts.

commonPage.ts should be updated to allow other content from htmlPage.ts
to be inserted into parts of the page.