css: Make message send / save buttons half opaque when disabled.

So far, the Send buttons area would turn grey when the message could not
be sent. The Save button when editing a message would also turn grey
when the message could not be edited anymore. Now we simply make the
buttons half opaque instead of turning them grey in a disabled state.

web/styles/compose.css

@@ -1110,14 +1110,7 @@ textarea.new_message_textarea,
 
         & button {
             pointer-events: none;
-        }
-
-        #compose-send-button {
-            background-color: hsl(0deg 0% 65%);
-        }
-
-        #send_later {
-            color: hsl(0deg 0% 65%);
+            opacity: 0.5;
         }
     }
 }

web/styles/dark_theme.css

@@ -104,8 +104,7 @@
         }
     }
 
-    .modal__btn:disabled,
-    .disabled-message-send-controls button {
+    .modal__btn:disabled {
         opacity: 1;
         background-color: hsl(0deg 0% 83% / 50%);
     }

web/styles/message_row.css

@@ -503,7 +503,6 @@
             /* Replicate the `.new-style` disabled values,
                without any color shifts. */
             cursor: not-allowed;
-            filter: saturate(0);
             opacity: 0.5;
         }
     }