fix links in prefixed sites built with the mass-build-sites pipeline

[gumaerc]

What are the relevant tickets?

Closes https://github.com/mitodl/ocw-studio/issues/2113

Description (What does it do?)

This PR takes the prefix argument being sent to SitePipelineDefinition and in addition to prefixing the S3 path with it, the Hugo baseURL argument is now also prefixed. This means that links rendered inside of sites deployed at a prefix URL will correctly link you to the prefixed page, at least for content generated by Hugo.

How can this be tested?

• Make sure you have run through the basic requirements for setting up ocw-studio and can publish sites locally
• Run docker compose exec web ./manage.py upsert_mass_build_pipeline --prefix test-prefix
• Browse to http://localhost:8080 and log into the Concourse web UI
• Find the prefixed mass build pipeline, unpause it and trigger the first batch. You can pause it after, as you only need to run through one batch to test this.
• In the batch's across step, find of of the sites it published and copy the url_path. Browse to the site by appending the url_path and the prefix to http://localhost:8045/. It should be something like http://localhost:8045/test-prefix/courses/site-name
• Click the links in the sidebar to browse the different parts of the site and verify that you stay under the test-prefix

[gumaerc]

@ibrahimjaved12 Thanks for your review. I looked into home page links, and I believe it's a bit beyond the scope of what this PR is doing. This PR simply sets the Hugo --baseUrl parameter to include the prefix. This means that links generated by Hugo (.Permalink, .RelPermalink, pageRef) will contain the prefix and work as expected. However, The links to the search page are mostly hard coded in ocw-hugo-themes to be relative to the root of the site. I think that in order to support this, we would need a PR in ocw-hugo-themes to be able to pull the prefix from an env variable and set it where necessary. This especially applies to the links within the search page itself, as the search page is a separate React app not generated by Hugo.

[gumaerc]

I created an issue here for adding support to the www theme for prefixed deployments: https://github.com/mitodl/ocw-hugo-themes/issues/1365