CI: Skip pipelines if no source files are changed

[henrybear327]

Based on some research (1 2 3 4 5) regarding job dependencies on GitHub CI:

• in order to define job dependencies, all jobs that have dependencies need to be defined in the same yml file
• there doesn't exist a way to cancel the started job gracefully

Thus, based on the current knowledge

• a job to test for code-related changes in certain directories will be the pre-req job for jobs that only need to be run if there are code-related changes
• because of the GitHub CI restrictions, jobs that require file-change checks are moved to mail.yml. By using dependencies, we can avoid adding if checks for all the steps that require file-change checks

Close #284