Change vue components `.js` files into `.vue` files

In order to aim for a more consistent code base and to eventually remove the template compiler from our production bundles we need to move our vue components in .js files into .vue files.

List of vue components in .js files:

• MR Widget components
• Issue boards components
• Cycle Analytics components - @filipa
• Diff notes components
• Filtered Search components
• Merge Conflicts
• Pipelines schedules
• Multiple Assignees
• Time tracking