[buildbot] support electroncash# and electrum# PR urls

Summary:
This should cause the following strings in commit messages to be turned into URLs when submitting a diff:

• electrum#2361
• electroncash#2081

Test Plan:

cd build/
cmake .. -GNinja
ninja check-buildbot

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14288