Add mention of unsplash in the background settings

src/components/lists/settings/background.vue

@@ -18,6 +18,7 @@
 						@keyup="() => newBackgroundSearch()"
 						:class="{'is-loading': backgroundService.loading}"
 				/>
+				<p class="unsplash-link"><a href="https://unsplash.com" target="_blank">Powered by Unsplash</a></p>
 				<div class="image-search-result">
 					<a
 							@click="() => setBackground(im.id)"

src/styles/components/list-backgrounds.scss

@@ -1,5 +1,14 @@
 .list-background-setting {
 
+  .unsplash-link {
+    text-align: right;
+    font-size: .8rem;
+
+    a {
+      color: $grey-dark;
+    }
+  }
+
   .image-search-result {
     margin-top: 1em;
     display: flex;