Don't replace wp.com account on Jetpack login

If there's already a wp.com account set up in the app, let's respect
that instead of replacing it.

Fixes #2259
Temporarily disable REST for Jetpack sites

It just doesn't work when the JSON API module is disabled.
Also, there are some issues where failed uploads don't show any error.
Enable Jetpack REST for debug/internal builds

fixes #2258
Drop the bracket :)
Merge pull request #2296 from wordpress-mobile/issue/2258-rest-jetpack-check

Enable Jetpack REST for debug/internal builds only
Merge pull request #2286 from wordpress-mobile/issue/2259-jetpack-login-respect-dotcom-account

Don't replace wp.com account on Jetpack login
Merge branch 'develop' into issues/2208-add-mentions-to-commenting