Load WMS tiles if EPSG:3857 (or equivalent) is supported by source

[tyrasd]

partially fixes #1141.

[bhousel]

This looks great @tyrasd - is it still a WIP?

It looks like you are planning to replace the '{proj}' with whichever supported projection editor-layer-index contains (currently hardcoded to 'EPSG:3857', which is the most common anyway).

[tyrasd]

is it still a WIP? It looks like you are planning to replace the '{proj}' with whichever supported projection editor-layer-index contains (currently hardcoded to 'EPSG:3857', which is the most common anyway).

Yeah, I'd like to still fix that hardcoded EPSG code there (there might be some obscure WMS services just supporting for example EPSG:900913 but not EPSG:3857).

Also, before we merge/release this, it would be good if we checked the respective WMS layers in ELI and add missing information like geographic extent – I think I saw quite a few of these claiming to be world-wide ("geometry": null) which actually aren't.

[tyrasd]

I'd say this can be merged now.

[bhousel]

Awesome @tyrasd - this is going to open up lots more imagery available for use in iD.. thank you!
I'm going to merge and add some test for the token replacement..