Change all hardcoded links to 18f.gsa.gov pages

[beechnut]

If you search the content folder for //18f.gsa.gov/, there are about 750 results.

At some point, we should change all of these links to the 11ty style of linking: {{ the path | url }}

Problem: In the absence of analytics, don't know if any of these links are broken, or how many, because we only check internal links, because checking external links is hard.

Benefit: If we change the linking from hardcoded to {{ }}, all of these links become internal links. (Right now, they're considered external when you're working locally, on localhost.) If these links become internal, they become check-able, improving the integrity of the site.

It's debatable whether this is in or out of scope for the migration, but since it's a non-trivial content change that doesn't block migration, I'm going to set it as out-of-scope for now.

### Tasks
- [ ] `content/` folder contains zero instances of `//18f.gsa.gov`
- [ ] Bonus points: a `npm run test:something` script checks each PR to make sure it stays that way

[bpdesigns]

straightforward and useful task

[beechnut]

@bpdesigns This is a post-migration task. This shouldn't be prioritized until after #3839 is complete.

[bpdesigns]

Thanks @beechnut I'll add this to blocked/waiting for now