Add activity column to pipeline security tab

What does this MR do and why?

Related #362575 (closed)

1. Add the activity column to pipeline security tab when feature flag pipeline_security_dashboard_graphql is enabled. At the moment this already allows the false positive detection of findings to be shown. The actual reason for this change is to later show related issues in this column with Show related issues on pipeline security tab (#433477 - closed) • David Pisek, Lorenz van Herwaarden • 16.7
2. Refactor the modal specs

Screenshots or screen recordings

Before	After

How to set up and validate locally

Prerequisites

1. You need an EE license
2. You need to have runners enabled (See $2408961 for setting up a runner)
3. Import https://gitlab.com/gitlab-examples/security/security-reports
4. Run a pipeline on master

Validation

1. Enable feature flag: echo "Feature.enable(:pipeline_security_dashboard_graphql)" | rails console
2. Go to the latest pipeline > security tab
3. Validate that the activity column is present
4. If there happens to be a finding which is a false-positive, then it will already show up in the activity column.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.