feature: email notification all stages

This pull request adds allows users to add email notifications for all stages of any given pipeline.

It tries to do this by introducing a new label called [ALL STAGES], returning it as part of the stages list when viewed via MyController. We've also updated the test cases and found the feature changes to work as expected to when tested locally. 

The choice for label "[ALL STAGES]" made sense to use, because its descriptive enough for the purpose and users are prohibited to create any piplines or stages with starting with special characters.