Adds default template text for issues and merge requests to the repository

What does this MR do?

The current text of issue and merge request template defaults are currently only available in the project settings. It should be more easily available and tracked with version control.

gitlab-org/gitlab-ce#58260 has been opened to address the automation of populating the default template text from a source file in the repository, so it will be necessary for a Maintainer or above to make the necessary changes if this MR is merged.

What are the relevant issue numbers?

gitlab-com/gl-security/engineering#244 gitlab-org/gitlab-ce#58260

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer