Improve image description user experience (#10036)

* Add image descriptions to searchable post content.

* Allow multi-line image descriptions.

* Request image descriptions in the same query as posts when creating the search index.

(see https://github.com/tootsuite/mastodon/pull/10036#discussion_r256551624)

app/javascript/styles/mastodon/components.scss

@@ -476,7 +476,7 @@
         opacity: 0;
         transition: opacity .1s ease;
 
-        input {
+        textarea {
           background: transparent;
           color: $secondary-text-color;
           border: 0;