Allow templates to be used for issues opened from alerts

We should allow for templates to be used in conjunction with the automatically opened issues. This would allow for all of the quick actions to be used, which would enable: at mentions, labels, assignees, and more.

This should:

• Provide an extensive amount of customization, leveraging existing features, without making our configuration UI overly complex
• Deprecate the existing mass email based approach, in favor of the at mention model in the template

This model can also be extended by the embedding framework we plan to build, allowing integration with a variety of services like Zoom, Slack, and more. https://gitlab.com/gitlab-org/gitlab-ce/issues/55757

Proposal

Allow an end user to select a template to use as a base for the issue that is created, under Settings->Operations->Incidents. For now, this could be a single dropdown to select.

When an issue is then created, the we can consider two options:

1. Add any incident specific content at the bottom of the template (easiest)
2. Support detecting some tag or delimiter, which can be replaced with the incident specific content.

This would include any embedded metrics charts, errors, or other information.