add noindex setting

[gumaerc]

What are the relevant tickets?

Closes https://github.com/mitodl/hq/issues/5393

Description (What does it do?)

This PR adds a new boolean setting to the app, MITOL_NOINDEX. This setting is meant to control the noindex meta tag in the head section of every page. It is true by default, preventing indexing of pages by search engines like Google. In production, we will set this value to false so that the tag is not written in and pages are indexed as expected.

How can this be tested?

• In your environment, set MITOL_NOINDEX=true (or omit the value, it defaults to true)
• Spin up this branch of mit-learn
• Visit the home page at http://localhost:8062/
• Inspect the page source and ensure that you see a <meta name="robots" content="noindex" /> had in the head section
• Change the value to false and reload your containers
• Visit the homepage and inspect the page source again, verifying that the tag is no longer present

Checklist:

• Merge and deploy set MITOL_NOINDEX var in mit-learn environments ol-infrastructure#2665

[abeglova]

👍