Step 1 - Clean up some collapsible_panel things

• Step 1 - Clean up some collapsible_panel things
• Step 2 - Extract sidebar_nav component from collapsible_panel
• Step 3 - Decouple resizeable_panel from collapsible_sidebar

What does this MR do?

Some various clean up to help decouple resizable_panel component:

• Adds helpful comment about jQuery usage in activity_bar.vue
• Moves multi-file-commit-panel class out of this components template root. Definitely not the responsibility of this component to set this. Especially given that it's sometimes passed multi-filecommit-panel-inner.
• Gives the multi-file-edit-pane a left and right border, which helps simplify some layout issues in upcoming right sidebar refactoring.

Screenshots

Everything still works 😄

Right sidebar collapsed	Right sidebar expanded

References

Related to #204821 (closed)