CRM-13872 - Anchor the regex so that isIssueKey does not feed invalid words to getIssue.

FIXME: It also seems correct to add ":" and "/" to the characters that split
words in AbstractWordFilter, but then the footnote begins to duplicate itself...
We probably need to check for existing footnotes in an earlier phase.
Merge pull request #1 from adamwight/CRM-13872

CRM-13872 - Anchor the regex so that isIssueKey does not feed invalid words to getIssue