Add support for other online XYZ sources

[rudokemper]

Goal

The goal of this PR is to add support for additional XYZ tile sources, and make it easier to add new ones in the future

What I changed

• Any code related to a concern of downloading XYZ tiles has been moved to its own file download_resources for legibility.
• Added a handler function requestOnlineTiles, which calls downloadOnlineTiles and returns a promise.
• downloadOnlineTiles takes a source and uses a URL associated with each to downloads XYZ tiles. In the case of Bing, quadkeys are used to get the tiles, but other sources (like Esri and Google) use the same endpoint structure so the function will JustWork™ with them.
• Terms of service for the supported services has been added.

Screenshots

Esri

Bing

Google

What I'm not doing here

Adding support for OSM. Per OSM's tile usage policy,

Bulk downloading is strongly discouraged. Do not download tiles unnecessarily.
In particular, downloading an area of over 250 tiles at zoom level 13 or higher for offline or later usage is forbidden. These tiles are generally not available (cached) on the server in advance, and have to be rendered specifically for those requests, putting an unjustified burden on the available resources.

I was not able to find an OSM mirror with a different offline tile usage policy, so I opted not to include OSM, and hence with this PR, will close #4.