Search sidebar aggregations UI doesn't match latest design

Created by: felixfbecker

Actual	Design

• Buttons should be individual buttons, not button groups
• "Repo" should say "Repository"
• "Grouped by" should say "Group results by"
• Bars should be limited in count and wider, so that no intermediate labels are skipped
• Initial zero state for the chart doesn't have "bars" background as it does in designs
• (minor) Corners are more rounded than in design
• (minor) There are no X axis ticks in design

Still missing:

• Info icon
• "+N" indicator

Other changes:

• "Save insights" button was cut from scope

/cc @joelkw @felixfbecker @vovakulikov @unclejustin