Fix spacing for branch switcher in pipeline editor

What does this MR do?

For #331689 (closed)

This makes sure that the top and bottom margins are the same and removes the left margin to align the branch switcher with the rest of the page.

The changes can be found in CI/CD > Editor on the sidebar.

Screenshots

Before	After

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• 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 performance guidelines)