1. All services are disabled in testing now to prevent network accesses
2. Only services are enabled when needed to test something
3. Many changes throughout code to allow iD to run with services disabled
(e.g. check for osm service instead of assuming context.connection() will work)
4. Actually export the services so we can disable and enable them
(closes#3324)
Previously we allowed devs to swap out services that they didn't need.
This became difficult now that ES6 exports are immutable bindings.
But we can wrap the immutable bindings themselves in a live object,
to get back the flexibility that we used to have.
This change also drops the `taginfo` accessor on Context, since devs who want
to swap out taginfo service with something else can now do so through the live
binding. `iD.services.taginfo = myTaginfo()`
