Flexbox buttons in top bar

- Makes the "Sidebar" toggle button permanent but removes the label
- Did some things to the "Save" button to make it the same width whether there
  is a count or not (prevents the buttons from jumping when pressing undo/redo)
- Removes a lot of the floated col rules that aren't used much anymore

data/core.yaml

@@ -296,7 +296,7 @@ en:
   help_translate: Help translate
   sidebar_button:
     title: Sidebar
-    tooltip: Open the sidebar.
+    tooltip: Toggle the sidebar.
   feature_info:
     hidden_warning: "{count} hidden features"
     hidden_details: "These features are currently hidden: {details}"