Add labels for assessing appsec sast quality

What does this MR do and why?

We've added labels appsec-sastcommented, appsec-sasthelpful, and appsec-sastunhelpful to help developers give efficient feedback on the AppSec SAST results. This is an alternative to writing long-form feedback on the feedback issue.

It will look something like this:

Closes Update AppSec SAST comment to be more helpful (gitlab-com/gl-security/product-security/appsec/sast-custom-rules#36 - closed)

Relates to Describe the labels we use to evaluate quality (gitlab-com/gl-security/product-security/appsec/sast-custom-rules!34 - merged)

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