Serve old sds-webapp2 home page from S3

[nickdos]

Child issue of #37.

Requirements

• serve the home page from sds.ala.org.au out of S3
• link to XML files also saved to s3
• include the last modified date/time of the sensitive-species-data.xml file

Challenges & solutions

• If we use static html (only), then keeping the ALA skin (header, footer, menus) up to date is a manual process that might get forgotten
  • Could be remedied with a React app that imports the header and footer from a common library, similar to how we're doing it in atlas-index
  • Would be served using same mechanism as static html
• layers.json is served from /ws/layers in old sds and is not a static file
  • Do we store a static json file or serve it from a different source?

[nickdos]

@adam-collins - do you have any thoughts on using static html and updating skin manually, when needed, versus creating a simple React app to pull the skin in and to have a build step that updates the source to deploy to S3 via an approval step?