Complete initial single method page

[echappen]

Changes proposed in this pull request:

• Completes layout for single method page—example on /methods/discover/cognitive-walkthrough
• Also fixes some duplicate package warnings in package.json

known issues

• We add link icons by evaluating markdown before it’s compiled into html. This doesn’t account for data-based html that doesn’t come from markdown, which is why some headers/links get the icon and others don’t. We should have a system that accounts for the compiled resulting html instead of the markdown, but I consider this outside the scope of this issue (I'll make an issue for it).
• We're using the same USWDS version and theme settings sitewide. Since the old Methods site uses USWDS v2x and this repo uses 3x, there will be slight differences in font size, spacing, etc.

Closes #228

security considerations

None

[echappen]

closing in favor of #273