Fixed Jetpack ID sync on REST

Added an extra check for `dotComID` before using REST, and enabled
Jetpack REST for internal builds again

Fixes #2705 


On readability and to modularize code a bit more, I'd suggest to put these conditions inside a method.  You can use any method (like `canUseJetpackREST` or similar).  My preferred change would be:

1. Change constant `WPJetpackRESTSupported` to `WPJetpackRESTEnabled`.
2. Change method `jetpackRESTSupported` to `jetpackRESTEnabled`.
3. Put these conditions inside a new `jetpackRESTSupported` method.

But this is just a personal opinion - the thing I'd definitely do is put these conditions inside a method.