Move RPC global strings to globals.h/cpp and access them through config

Summary: Prep for T401. Completes T329.

Test Plan: make check && test_runner.py

Reviewers: deadalnix, Mengerian, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1731