Resolve "Stop markdown caching of non-markdown Snippet content"

What does this MR do?

This MR changes the way the CacheMarkdownField concern performs a cache refresh, in that it now checks if the field can be cached first. By default it will be true but in the case of Snippets and the content field, it will now be conditional based upon the Snippet file_name.

This change is being made because previously the concern would cache the HTML generated from parsing all fields for the model as if they were markdown content. For Snippets, this is not always the case. The content field is not always markdown and in #204894 (closed) this would actually cause a stack error.

Additional benefits of this change is that we will now no longer be parsing and storing content in the DB unnecessarily.

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

Closes #207976 (closed)