Fix font-size for organization landing content heading

[mcbouslog]

Staging branch URL: https://pr-5590.pfe-preview.zooniverse.org

• staging: https://pr-5590.pfe-preview.zooniverse.org/organizations/markb-panoptes/urban-wildlife-org-test
• production: https://pr-5590.pfe-preview.zooniverse.org/organizations/md68135/notes-from-nature?env=production

Some of the last edits to the recent org landing redesign PR #5554 included adding or editing header elements for content and section headings. These edits changed the font-size for some headings, which previously were set on default or inherited sizing. This PR change explicitly sets the font-sizes in accordance with the design.

Required Manual Testing

• Does the non-logged in home page render correctly?
• Does the logged in home page render correctly?
• Does the projects page render correctly?
• Can you load project home pages?
• Can you load the classification page?
• Can you submit a classification?
• Does talk load correctly?
• Can you post a talk comment?

Review Checklist

• Does it work in all major browsers: Firefox, Chrome, Edge, Safari?
• Does it work on mobile?
• Can you rm -rf node_modules/ && npm install and app works as expected?
• If the component is in coffeescript, is it converted to ES6? Is it free of eslint errors? Is the conversion its own commit?
• Are the tests passing locally and on Travis?

Optional

• Have you replaced any ChangeListener or PromiseRenderer components with code that updates component state?
• If changes are made to the classifier, does the dev classifier still work?
• Have you resized and compressed any image you've added?
• Have you added in flow type annotations?
• Have you followed the Springer guidelines for commit messages?

[coveralls]

Coverage remained the same at 49.352% when pulling f61b58d on org-landing-heading-style into 2d81e61 on master.

[mcbouslog]

Closing in favor of new PR that addresses additional items as noted in #5592