Suggested Solution - Auto-fix MRs - backend

This is a sub-issue for #216027 (closed)

We need to create auto-fix MR if the setting for auto-fix is enabled:

Implementation plan

• Register the bot user GitLab-security-bot that will author auto-fix MRs.
  • When registering the bot, we should set a status that will make it clear that it's system bot: :robot: System user bot
• Make GitLab-security-bot's profile link available to the frontend (HAML).
• Automatically create remediation MRs assigned to GitLab-security-bot and labelled with gitlab-vulnerability-auto-fix whenever an available solution is detected for a given vulnerability.
• Notify the users that MRs were created via an alert/flash message (picture e.).

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