Add ability to enter number in related issues

What does this MR do?

This MR gives the ability to type an issue number and then will prepend a # sign to the beginning because this triggers the autocompleted library to do our search.

More context here: #233975 (comment 413125164)

Repro: Go to any issue, go to the related issues section, add a related issue by just typing your issue number. note: pasting issue numbers is currently not working but will be addressed in a follow up. Ie pasting 123 into the input field.

Relates to #233975 (closed)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team