Maptiler custom map ids are only useable by the account that create them. So if we hardcode them forkers won't be able to use the maps even if the bring in their own api key (because they can't access our maps with their api key).
Requires to set our map ids in `local.properties` for local development:
```
services.maptiler.lightMapId=9bc819c8-e627-474a-a348-ec144fe3d810
services.maptiler.darkMapId=dea61faf-292b-4774-9660-58fcef89a7f3
```
In an effort to make it easier for forks to (a) use their own
API keys (b) change map styles or maybe even providers, move
the MapTiler key out of the source code and pass it in via
env var or property.
Also refactor the utility classes slightly to keep all the URL
related functions together, to reduce the chance of collisions
when maintaining such forks.
Show a fully-featured MapView, centered on the dropped pin,
which allows panning/zooming. Share button allows opening
in a map application.
Supports showing a description at the top of the screen,
if one is supplied with the event.
Out of scope: showing the local user's location (being
done as a separate story).
Includes some minor tidying: remove duplicate Location,
and make GeoURI parsing a method on that class; fix the
pointer location in MapView (I broke it earlier, whoops!)
