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/mastodon/features/compose/components/upload.js

@@ -108,9 +108,8 @@ class Upload extends ImmutablePureComponent {
                 <label>
                   <span style={{ display: 'none' }}>{intl.formatMessage(messages.description)}</span>
 
-                  <input
+                  <textarea
                     placeholder={intl.formatMessage(messages.description)}
-                    type='text'
                     value={description}
                     maxLength={420}
                     onFocus={this.handleInputFocus}