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/views/components/code-editor.blade.php

@@ -43,7 +43,7 @@
                 </div>
 
                 <div class="form-group">
-                    <button type="button" class="button primary" @click="save()">{{ trans('components.code_save') }}</button>
+                    <button type="button" class="button" @click="save()">{{ trans('components.code_save') }}</button>
                 </div>
 
             </div>