Implement intro screen transition

[caseycesari]

Overview

Implements a transition from the intro screen to the home screen, as shown in the prototype, via the following changes:

• The background fill on the intro screen was eliminated so that the map is visible.
• A new map style was created for the intro screen that does not have any labels. When transitioning, this style is swapped out for the style with the labels.
• The sensor markers are only added to the map after the "Find out" button is clicked.
• The map pitch is adjusted as the transition is happening.

Additionally, the style of the intro screen was adjusted to remove a "hiccup" when transitioning to the home screen style due to use of absolute positioning.

Connects #82

Demo

Due to the map style, GIFs are very large and can't be uploaded to Github. Check out the Netlify deploy preview.

Testing Instructions

• Visit the app, and verify the intro screen map meets the conditions described in the issue.
• Click the "Find out" button, and verify the map transitions, the markers are added, and the map labels are displayed.

[caseycesari]

Because the map styles are switched when the transition starts, the change in map pitch doesn't look as good as it could if the map style stayed the same. However, having labels on the intro screen map doesn't work very well with the text. @alexelash Looking for some input here. I couldn't find a way to show/hide the labels dynamically without publishing a separate map style.

[caseycesari]

Thanks!