Sanitize change request identifiers

Change request identifiers are currently entirely based on the title by default.
However we should find an implementation that guarantee:
1. to keep part of the title so that it remains human readable
2. to be unique to not have collision
3. to limit the set of characters used (we should probably only allow alpha numeric characters)

The third condition is particularly important since we're relying on patterns at several places, which contains change request identifiers.