Adding SAST template should change to an "include"

Problem to solve

With the current workflow of Add File -> select .gitlab-ci.yml -> select template SAST, all of the contents of SAST.gitlab-ci.yml get copied in to gitlab-ci.yml. All of the SAST variables are being overwritten with what are our defaults as of that time, and the user wouldn't receive any updates we push to SAST.gitlab-ci.yml.

Solution

• Adding the SAST template in Files adds an include as specified in the docs.
• Consider adding a comment encouraging configuration in the SAST Config UI (not shown below)

Current	Proposed

This change also matches the experience of adding the Code Quality template.

Implementation plan

• Move Security/Secret-Detection.gitlab-ci.yml and Security/SAST.gitlab-ci.yml to Jobs
• Add Security/** placeholders that include the moved Job/** templates

cc @tmccaslin @rossfuhrman @theoretick @ssarka @twoodham @nmccorrison