ui: Sidebar stylistic improvements

[dmohns]

This PR introduces some stylistic improvements to the sidebar to make it less confusing.

• Change the sub-list expand arrow colour to match the font colour. Before it was black and dark grey background and really hard to see.
• Consistently position the sub-list expand arrow to the right of the menu item. Before it's position was inconsistent due to a cross reference from CategoryManagement.vue
• Change the "active selection highlight" to a tinted, right border. Before highlighting was achieved by moving the selected item to the left. Moving items feel like bad UX and it doesn't really work well for sub-menu items.
• Adds a small lock indicator 🔒 when a page or sub page is password protected

Preview:

Before: