Welcome modal and endpoint notice

[jthiller]

Closes #25

This adds a dismissable welcome modal to mappers. The modal currently opens on each visit so I'd like to do a fast-follow with some functionality around remembering the state of the modal.

TODO:

• [x] Serve background SVG locally. Webpack is currently complaining about svg
• [x] Set final route for endpoint change guide. Currently pointing to https://docs.helium.com/use-the-network/coverage-mapping

[kent-williams]

looks and works great!