Be able to do Github hotlinking to repos which aren't bridged

Ye olde goneb used to hotlink anything using the github style cross-repo mentions, i.e. `matrix-org/synapse#1000` hookshot doesn't do any hotlinking by default and doesn't support linking to non-bridged repos.

Start by tracing the GitHub cross-repository mention handling in matrix-hookshot and how the bridge decides which repositories can be linked. Confirm the expected URL behavior for references such as `matrix-org/synapse#1000`; the work is done when GitHub-style references to non-bridged repositories hotlink correctly without requiring a bridge.

Written by the indexing model from the issue text.