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/ui/layout/AboutDialog.vue

@@ -1,7 +1,7 @@
 <template>
 <div class="c-about c-about--splash">
     <div v-if="branding.aboutHtml" class="s-text l-content" v-html="branding.aboutHtml"></div>
-    <div v-else class="c-about__image"></div>
+    <div v-else class="c-about__image c-splash-image"></div>
 
     <div class="c-about__text s-text">
         <h1 class="l-title s-title">Open MCT</h1>