Block save button when uploading a send (#871)

* Block save button when uploading a send

* Add overflow hidden to prevent scrollbar appearing

src/scss/buttons.scss

@@ -86,3 +86,29 @@
         }
     }
 }
+
+.btn-submit {
+    position: relative;
+    overflow: hidden;
+
+    .fa-spinner {
+        position: absolute;
+        display: none;
+        align-items: center;
+        justify-content: center;
+        bottom: 0;
+        top: 0;
+        left: 0;
+        right: 0;
+    }
+
+    &:disabled:not(.manual), &.loading {
+        .fa-spinner {
+            display: flex;
+        }
+
+        span {
+            visibility: hidden;
+        }
+    }
+}