Add search and high-contrast mode to DRC site

[feralresearch]

What this does

Addresses #7
Addresses #5
NB: I realize this is two distinct features and should have been two PRs, mea culpa

1. Adds Lunr search to DRC site
2. Adds high-contrast visual mode

Also: fixes a loading glitch where the submenus would sometimes appear and then disappear.

How to test

NOTE: Search doesn't need testing here, it's been superceeded by a different PR

Search:
1. Enter search term on search box on main page and hit "enter"

High Contrast:

1. Click the "toggle contrast" button on the menu, observe the change
2. Browse the site. Please report anything that looks broken or not high-contrast enough
3. Please confirm the setting "sticks" even if you close the browser and return later

[SteveMarvin]

I need a PR link here.

[SteveMarvin]

There are a few scattered items that are not shifting out of the gray font (such as "Author"), but they are switched to a white background, which seems sufficient. ACCEPTED, with the caveat that the search functionality is not in yet.