only add routes for email store if plugin enabled

I had issues with that (seems like express didn't like the order of the routes declaration). This PR is here because the insight web frontend didn't liked that.

See: https://github.com/bitpay/insight-api/commit/29d8e32120a4cbd7f4545f4f00037611d922f6d1#diff-862b29769cba9727c338a8dc473b1379L101