[buildbot] make bot link to backports of core-gui repo

Summary:
the idea is to have a markdown convention that will link to the two repos we already backport from and include the bitcoin-core/gui repo

after this diff I establish the following convention:

summary text in the format LOWERCASE_SUPPORTED_REPO_PREFIX#PR_NUMBER will be replaced with a link to the appropriate PR if LOWERCASE_SUPPORTED_REPO_PREFIX is in the relevant dictionary in server.py

examples:

core#17234 should replace to core#17234

core-gui#9 to core-gui#9

and secp256k1#894 to secp256k1#894

Test Plan:

pytest

Reviewers: Fabien, #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9258