Fix token recognition for github issues
Fix token recognition for github issues

Fixes the issue where the regex was matching URLs and such in messages, making
chatbot give error messages saying that a project was not found. The match now
requires the github issue string to be preceded by a whitespace (or start of the
line) and followed by a whitespace (or end of the line).

dssit/dss-chatbot-slack,markediez/dss-chatbot-slack,ucdavis/aggie-bot