Modify encrypted images loading for better UX

Adds proper react css transition to encrypted images placeholders, so
that we can animate it's entrance and exit properly.
In addition, adds simple css animations file to properly handle all of
the animations in one place, so that it's much easier to properly handle
prefers-reduced-motion media query.

res/css/_animations.scss

@@ -0,0 +1,57 @@
+/*
+Copyright 2021 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+/**
+ * React Transition Group animations are prefixed with 'mx_rtg--' so that we
+ * know they should not be used anywhere outside of React Transition Groups.
+*/
+
+.mx_rtg--fade {
+    &-enter {
+        opacity: 0;
+        &-active {
+            opacity: 1;
+            transition: opacity 300ms ease;
+        }
+    }
+    &-exit {
+        opacity: 1;
+        &-active {
+            opacity: 0;
+            transition: opacity 300ms ease;
+        }
+    }
+}
+
+
+@keyframes mx--anim-pulse {
+    0% { opacity: 1; }
+    50% { opacity: 0.7; }
+    100% { opacity: 1; }
+}
+
+
+@media (prefers-reduced-motion) {
+    @keyframes mx--anim-pulse {
+        // Override all keyframes in reduced-motion
+    }
+    .mx_rtg--fade-enter-active {
+        transition: none;
+    }
+    .mx_rtg--fade-exit-active {
+        transition: none;
+    }
+}

res/css/_common.scss

@@ -18,6 +18,7 @@ limitations under the License.
 
 @import "./_font-sizes.scss";
 @import "./_font-weights.scss";
+@import "./_animations.scss";
 
 $hover-transition: 0.08s cubic-bezier(.46, .03, .52, .96); // quadratic
 

res/css/views/messages/_MImageBody.scss

@@ -16,6 +16,12 @@ limitations under the License.
 
 $timelineImageBorderRadius: 4px;
 
+.mx_MImageBody_thumbnail--blurhash {
+    position: absolute;
+    left: 0;
+    top: 0;
+}
+
 .mx_MImageBody_thumbnail {
     object-fit: contain;
     border-radius: $timelineImageBorderRadius;
@@ -23,8 +29,11 @@ $timelineImageBorderRadius: 4px;
     display: flex;
     justify-content: center;
     align-items: center;
+    height: 100%;
+    width: 100%;
 
-    > div > canvas {
+    .mx_Blurhash > canvas {
+        animation: mx--anim-pulse 1.75s infinite cubic-bezier(.4, 0, .6, 1);
         border-radius: $timelineImageBorderRadius;
     }
 }