Isolating MAX_COINBASE_SCRIPTSIG_SIZE

Summary:
The constant MAX_COINBASE_SCRIPTSIG_SIZE was badly located in
'miner.cpp' and thus moved to 'consensus.h'. Furthermore, the
validation was relying on an inline magic number which is now
replaced by the constant.

Test Plan: Compile and run automated tests.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Subscribers: deadalnix, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1603