Adding support for v2 api version.

[alexcottner]

Work in progress...

Need to:

• Add support for v2 routes in dev-tools
• Add support for v2 routes in kinto-http-client (link)
• Publish new kinto-http-client to firefox (link)
• Figure out a reasonable UI for this
• Update tests

[leplatrem]

If we want to gradually migrate our clients from v1 to v2, shouldn't we have a pref on the client instead, and do there what you do here in the devtools:

const apiVersion = Services.prefs.getCharPref("api_version", "/v1");
const serverUrl = new URL(SERVER_URL, apiVersion);

[alexcottner]

shouldn't we have a pref on the client instead

I was thinking leaving that as a single pref in services.settings.server and allowing it to be overridden in it's entirety. That feels simpler or am I wrong?

Splitting it in two for the dev-tools UI just feels easier because instead of a single dropdown with 14 options we have two dropdowns. (and if/when we make a v3 it would be 21)