Resurrect section html theming, custom layouts w/ fallback

[isaacwhite]

Description of Change

This PR makes two changes to the way our templates work to support custom layouts.

1. It extends ejs view resolution logic to search custom/layouts first, before the default layouts folder
2. It updates the getTemplates() utility function to attempt to search this custom/layouts directory as well.

It also changes all the partial includes to not use relative paths, which means custom partials can also be resolved from a custom/layouts folder.

This change makes preexisting logic that allowed custom category templates based on the root path of Library urls more useful, because now you could add a custom/layouts/categories folder with your-base-url-segment.ejs and override the root template for subcategories of your Library site.

Related Issue

#45 #36

Motivation and Context

There are a few exciting possibilities that custom HTML opens up for Library sites. This change represents a first pass at reenabling some customization functionality that was envisioned since the beginning, and empowering custom folder overrides to also change the HTML of any portion of the Library page, which was a missing feature at the time we originally open sourced the project.

Checklist

• Ran npm run lint and updated code style accordingly
• npm run test passes
• PR has a description and all contributors/stakeholder are noted/cc'ed
• [ ] tests are updated and/or added to cover new code
• relevant documentation is changed and/or added

[isaacwhite]

cc @thebestsophist and @knowtheory, who asked about this over in #proj-library.

[afischer]

👍
We should be sure to update the documentation about this as well.