Add template install and update scripts

[lorenyu]

Ticket

Resolves #173

Changes

• Add download-and-install script
• Add install-template script
• Add update-template script
• Add template-only-cd.yml workflow

Context

Based on similar scripts in template-infra and template-application-nextjs so that we don't have to manually apply changes to the test repos (e.g. platform-test-flask) every time there's a change to the template.

For similar PR on nextjs see https://github.com/navapbc/template-application-nextjs/pull/93

Testing

• Manually triggered the template deploy workflow. There were no diffs so no commit was created.
• So I introduced a trivial change in commit a2a6a01 and triggered the workflow again, which produced this commit on the platform-test-flask repo