Hides the merged YAML tab content when a non-default branch is selected

What does this MR do?

When a non-default branch is selected, the merged YAML result is potentially incorrect (#332845 (closed)). This is an issue that's being addressed elsewhere, but a potential boring solution for now is to hide the merged yaml result when editing on the non-deafult branch. This is what this MR does.

It's a temporary solution that should be reverted as soon as the merged YAML functionality works correctly across multiple branches.

Screenshots or Screencasts (strongly suggested)

Note: The video below showcases an older approach. Now we show the tab, but display the message shown in the above screenshot.

yaml

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• [-] I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [-] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request perxormance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.