Resolve "Add system notes for when a zoom call was added/removed from an issue"

What does this MR do?

Adds a system note when a loom link is added/removed to an issue description.

video example:

Screen_Recording_2019-07-20_at_12.20.45

Notes for the reviewer

There is a limitation in this implementation: if the issue description already has a zoom link and we add another zoom link no note is shown. A similar issue occurs when we have 2 zoom links in the issue description and we remove only one link: in this case no message is shown.

I think this should be acceptable as first iteration since also the "Pinned zoom link icon" doesn't handle multiple links as well.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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

Closes #63547 (closed)