Converted primary color use to css variable

- Removed all existing SCSS usage of primary color.
- Cut down custom styles injection to just be css vars.
- Reduced button styles so default button is primary.
- Updated button styles to lighten/brighten on hover & active states even
when a custom color is set.
- Removed unused scss color vars.
- Updated default BookStack blue to achieve better accessibility.

resources/assets/js/components/setting-app-color-picker.js

@@ -10,7 +10,7 @@ class SettingAppColorPicker {
         this.colorInput.addEventListener('change', this.updateColor.bind(this));
         this.colorInput.addEventListener('input', this.updateColor.bind(this));
         this.resetButton.addEventListener('click', event => {
-            this.colorInput.value = '#0288D1';
+            this.colorInput.value = '#1d75b6';
             this.updateColor();
         });
     }