Use CDN domains for asset prefixes

[eatyourgreens]

• use www.zooniverse.org and frontend.preview.zooniverse.org for asset prefixes in production and staging, respectively.
• move project images to www.zooniverse.org/projects/assets/ so that we can route image URLs to the NextJS app.
• Add robots.txt to ban bots from fe-project.zooniverse.org and fe-content-pages.zooniverse.org

Package:
app-content-pages
app-project

Closes #715
Closes #2518

Depends on the nginx config being updated first, to route the asset prefix URLs to the NextJS apps.
zooniverse/static#253

Review Checklist

General

• Are the tests passing locally and on Travis?
• Is the documentation up to date?

Components

• Has a storybook story been created or updated?
• Is the component accessible?
  • Can it be used with a screen reader? BBC guide to testing with VoiceOver
  • Can it be used from the keyboard? WebAIM guide to keyboard testing
  • Is it passing accessibility checks in the storybook?

Apps

• Does it work in all major browsers: Firefox, Chrome, Edge, Safari?
• Does it work on mobile?
• Can you yarn panic && yarn bootstrap or docker-compose up --build and app works as expected?

Publishing

• Is the changelog updated?
• Are the dependencies updated for apps and libraries that are using the newly published library?

Post-merging

• Did the app deploy to https://frontend.preview.zooniverse.org/projects/:project-name/:owner or https://frontend.preview.zooniverse.org/about?
• Is the new feature working or bug now fixed?
  • Is there a Talk or blog post written to announce the new feature(s)?
• Is the design working across browsers (Firefox, Chrome, Edge, Safari) and mobile?
  • Is this approved by our designer?
• Is this ready for production deployment?

[camallen]

LGTM - is the prod CDN all setup to handle this change?

[eatyourgreens]

zooniverse/static#253 set up the URLs that are used here, but we need to test this out on staging before changing the production FrontDoor setup. First step, after this PR merges, should be to remove the rule for frontend.preview.zooniverse.org/projects/* in FrontDoor and check that the nginx rules work as expected.