Replace leaflet map with static image in welcome page

[jacobtylerwalls]

Visiting /welcome and then /draw fetches tiles for a <Map> component twice.

We thought this was addressed in 3e0f1c7, but I wasn't able to verify that, and even if solved at one point, it probably was re-caused in #77. Further, adding other non-map routes such as /submissions makes it difficult to conditionally render a single <Map> object for certain routes only.

Proposed solution: replace the <Map> in the welcome page with a static background image.