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/auth/passwords/email.blade.php

@@ -16,7 +16,7 @@
                 </div>
 
                 <div class="from-group text-right mt-m">
-                    <button class="button primary">{{ trans('auth.reset_password_send_button') }}</button>
+                    <button class="button">{{ trans('auth.reset_password_send_button') }}</button>
                 </div>
             </form>
 

resources/views/auth/passwords/reset.blade.php

@@ -26,7 +26,7 @@
                 </div>
 
                 <div class="from-group text-right mt-m">
-                    <button class="button primary">{{ trans('auth.reset_password') }}</button>
+                    <button class="button">{{ trans('auth.reset_password') }}</button>
                 </div>
             </form>