Fix Template Credentials in the UI / Turn on typechecking of UI code

[foot]

Closes #3703

What changed?

• Turns on typechecking of UI code in CI
• Fix up type errors (including a fix for Template credentials do not load in the UI #3703 )

Why was this change made?

• We were not typechecking in CI
• So we missed a big bug.

How was this change implemented?

• types types types

How did you validate the change?

• Check ui-unit-tests job, it now typechecks as well as lints

Manually:

• SKIP_UI_BUILD=1 tilt up
• yarn start
• nav to template > use this template (http://localhost:3000/templates/create?name=vcluster-template-development&namespace=default) Note that credentials dropdown appears (even though empty, it at least appears now)

[opudrovs]

I don't have context for this 🐛 (except for the issue description), but code LGTM.