Merge branch 'media-preview-fit' into 'develop'

Letterbox media upload previews again

See merge request soapbox-pub/soapbox-fe!515

app/styles/components/compose-form.scss

@@ -299,6 +299,7 @@
     .compose-form__upload-thumbnail {
       border-radius: 4px;
       background-position: center;
+      background-size: contain;
       background-repeat: no-repeat;
       height: 160px;
       width: 100%;