Refactor GerritHookfilter's issue extraction into separate class

This refactoring brings the functions to the new class as unmodified
as possible. Thereby, we allow easier tracing of changes. Refactoring
of the code is left to follow-up commits.

After some more refactoring, we use this class as single way to
extract issue ids from commit messages.

Change-Id: I2d1dcab2cabc4992c975f649c167d8511cc1f0ab