Reword link preview settings to better match reality

- s/Links and URLs/Link previews/: From a user perspective, these are the same thing... These options are for the prefetcher, not the URLs themselves
- s/thumbnails/images/: What we call thumbnails are for "site" previews, not actual image links
- s/links/websites/: Technically, both image and non-image links are links, "websites" carries a tiny bit better the meaning

client/index.html

@@ -282,18 +282,18 @@
 							</div>
 							{{#if prefetch}}
 							<div class="col-sm-12">
-								<h2>Links and URLs</h2>
+								<h2>Link previews</h2>
 							</div>
 							<div class="col-sm-6">
 								<label class="opt">
 									<input type="checkbox" name="thumbnails">
-									Auto-expand thumbnails
+									Auto-expand images
 								</label>
 							</div>
 							<div class="col-sm-6">
 								<label class="opt">
 									<input type="checkbox" name="links">
-									Auto-expand links
+									Auto-expand websites
 								</label>
 							</div>
 							{{/if}}