Let users save locations and factor js into a few modules

[kalilsn]

Opening a draft for some feedback now, but I'll update this again when I finish making the changes that Jean has already asked for:

• [x] Since this disables double click zoom, only enable it if a checkbox is selected (with an explanatory tooltip)
• [x] Change the way we handle geolocation so that:
  • there's always a marker at the user's location
  • if the user uses their position as the source/destination, that marker stays static after a search (while the user location marker continues to move)

I'll also add a new style for the location marker and lightly clean up the styles on the new popup elements I've added.

Aside from UI questions, I'm also interested in feedback on the overall structure here. For example, there's definitely more stuff that could be broken out into its own module to shrink app.js, let me know if there's something there I should do. Should the modules be more functional instead of classes? Should the modules be organized differently? Should we implement this like a redux app with state + reducers?

Closes #35

[kalilsn]

I made two of the changes you asked (darker location marker and hearts for saved locations) and I'll open issues for the rest, then merge this.

I appreciate your thoughts about organization: I also don't know what will work best and I'm just trying things out! Grouping by feature seems reasonable so that's what I'll stick with for now.