Fix legacy messages (#2323)

* Fix legacy message styling

- Code cleanup;
- Enable constants-mobile;
- Add _legacy-messages.scss;
- Add status color fg colors to theme constants;

* CSS refactoring, significant migration for messages classes

- Many messages classes migrated;
- c-click-icon > c-icon-button;
- c-click-icon rewritten;
- __close-btn refined in dialogs;

* About and splash screen styling

- Fixed splash in About screen;

* Update _mixins.scss

* Convert c-overlay__close-button to button

- <a> -> <button>;
- Set color of button;
- Normalized naming from close-btn to close-button;

* Fixed brightness filter issue on elements in overlays in VISTA;

* Fix dismiss button

src/plugins/notebook/res/templates/entry.html

@@ -26,7 +26,7 @@
     </div>
 
     <div class="c-ne__local-controls--hidden">
-        <button class="c-click-icon c-click-icon--major icon-trash"
+        <button class="c-icon-button c-icon-button--major icon-trash"
            title="Delete this entry"
            @click="deleteEntry">
         </button>