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>

resources/views/components/entity-selector-popup.blade.php

@@ -7,7 +7,7 @@
             </div>
             @include('components.entity-selector', ['name' => 'entity-selector'])
             <div class="popup-footer">
-                <button type="button" disabled="true" class="button entity-link-selector-confirm primary corner-button">{{ trans('common.select') }}</button>
+                <button type="button" disabled="true" class="button entity-link-selector-confirm corner-button">{{ trans('common.select') }}</button>
             </div>
         </div>
     </div>

resources/views/components/image-manager.blade.php

@@ -72,7 +72,7 @@
                                     <button type="button" class="button icon outline" @click="deleteImage">@icon('delete')</button>
 
                                 </div>
-                                <button class="button primary anim fadeIn float right" v-show="selectedImage" @click="callbackAndHide(selectedImage)">
+                                <button class="button anim fadeIn float right" v-show="selectedImage" @click="callbackAndHide(selectedImage)">
                                     {{ trans('components.image_select_image') }}
                                 </button>
                                 <div class="clearfix"></div>