various UI fixes

* hide borders of images in bg when in the message details view
* force color of caption edits to white, as the background is grey and
it is more readable
* fix a bug preventing to quote the same message twice when switing
between two conversation after starting to quote one

stylesheets/_modules.scss

@@ -2254,6 +2254,7 @@
     border-radius: 100px;
     #session-input-floating-label {
       padding: $session-margin-sm;
+      color: white;
     }
 
     label.session-input-with-label-container.filled {

stylesheets/_session_conversation.scss

@@ -150,7 +150,7 @@
     align-items: center;
     height: 100%;
     width: 100%;
-    z-index: 1;
+    z-index: 5; // to be sure to hide the borders of images in messages
     background-color: inherit;
     display: none;
     padding: 20px;

ts/components/session/conversation/SessionConversation.tsx

@@ -217,6 +217,7 @@ export class SessionConversation extends React.Component<Props, State> {
         isDraggingFile: false,
         messageDetailShowProps: undefined,
         quotedMessageProps: undefined,
+        quotedMessageTimestamp: undefined,
       });
     }
   }