Add existing flag for Discover Movie Posters

frontend/src/DiscoverMovie/Posters/DiscoverMoviePoster.css

@@ -68,6 +68,19 @@ $hoverScale: 1.05;
   color: var(--white);
 }
 
+.existing {
+  position: absolute;
+  top: 0;
+  left: 0;
+  z-index: 1;
+  width: 0;
+  height: 0;
+  border-width: 25px 25px 0 0;
+  border-style: solid;
+  border-color: #37bc9b transparent transparent;
+  color: var(--white);
+}
+
 .controls {
   position: absolute;
   bottom: 10px;

frontend/src/DiscoverMovie/Posters/DiscoverMoviePoster.css.d.ts

@@ -7,6 +7,7 @@ interface CssExports {
   'controls': string;
   'editorSelect': string;
   'excluded': string;
+  'existing': string;
   'externalLinks': string;
   'link': string;
   'overlayTitle': string;

frontend/src/DiscoverMovie/Posters/DiscoverMoviePoster.js

@@ -159,6 +159,14 @@ class DiscoverMoviePoster extends Component {
               />
           }
 
+          {
+            isExisting &&
+              <div
+                className={styles.existing}
+                title={translate('Existing')}
+              />
+          }
+
           <Link
             className={styles.link}
             style={elementStyle}