[EPIC] Post-migration tasks

[beechnut]

### Tasks
- [ ] #3862
- [ ] Social media sidebar links aren't getting fixed by our custom markdown renderers. Figure this out and refactor our renderers so they're more legible/maintainable
- [ ] Remove all the site.baseurls, using image shortcodes or the URL filter wherever they remain
- [ ] Convert blog post images from `<img>` tags to image shortcodes
- [ ] Add code block line numbers
- [ ] /tags is a weird page
- [ ] Tags are inconsistently capitalized (see sidebar in /blog vs /tags)
- [ ] Add validations in a pre-build step. e.g. only two guides can have `featured: true` at a time; all pages should have layouts specified; front matter and path dates must match; six projects must be pinned in order; ...
- [x] Let us conditionally hide blog post banners, esp for smaller-width images - (@beechnut fixed when re-implementing `hero: false`)
- [ ] Add validation: Prevent h1s in blog post source markdown, see [#4006](https://github.com/18F/18f.gsa.gov/pull/4006/), https://github.com/18F/TLC-crew/issues/884
- [ ] Un-ignore ESLint comments, and/or switch the site and 18F/guides to standardjs
- [ ] Speed up build by optimizing data lookups. Instead of doing it dynamically in templates, set up some JS in the pre-build stage to cache data values that you can just read in the template (caution: this is making a lot of assumptions about where the build is slow)
- [ ] Fix all width-\d+ classes in _sass/components/utilities.scss [link](https://github.com/18F/18f.gsa.gov/pull/3863/files#diff-4fcf2969d7569fe0bf207a9f3df7c19e9f19e263bcb1265ec618435f7788ab95R72)
- [ ] Reconsider the image for the Alaska DHSS case study
- [ ] Link to partnership principles from footer, /about, /our-work
- [ ] https://github.com/18F/18f.gsa.gov/issues/3973
- [ ] Add profile photos to author pages when available, opt-in
- [ ] https://github.com/18F/18f.gsa.gov/issues/3977
- [ ] Uninstall Eleventy Navigation plugin.
- [ ] https://github.com/18F/TLC-crew/issues/862
- [ ] Consider setting up a `revisit` frontmatter variable for posts we want to update and revisit, like Melody Kramer's technical tutorials, so the build alerts when it's been a year since the last update
- [ ] Test site setup with dev-adjacent folks (product designers etc)
- [ ] https://github.com/18F/18f.gsa.gov/issues/3860
- [x] Refactor all the files that contribute to config in .eleventy.js. Anything that can move into another file (e.g. `config/collections.js`) should.

[bpdesigns]

@beechnut please turn these tasks into individual tickets with acceptance criteria so we can prioritize these. Thank you!

[beechnut]

@bpdesigns I'm of the opinion that these shouldn't be addressed or prioritized until the migration is fully complete. Thoughts?

[bpdesigns]

makes sense to me. I'll add them to the icebox for now. Thanks!

[beechnut]

@amandacostello I moved this into "Candidates for prioritization" now that the replatforming is done.