Fix #4405: Post version table css doesn't keep ratio at smaller resolutions.

Bug: extremely long sources cause the Tags column to become extremely
wide. Caused by the source link not having the word-break property set.

Fix: use post_source_tag, which lets the `a[rel=external] { word-break: break-word; }`
rule take effect.

Example: https://danbooru.donmai.us/post_versions?search%5Bpost_id%5D=3809742

app/views/post_versions/_listing.html.erb

@@ -24,7 +24,7 @@
         </div>
         <div><b>Tags:</b> <%= TagSetPresenter.new(post_version.tag_array).inline_tag_list_html %></div>
         <div>
-          <%= post_version_field(post_version, :source) %>
+          <%= post_source_tag(post_version.source) %>
         </div>
       <% end %>
       <% t.column "Edits", td: {class: "col-expand"}, width: "40%" do |post_version| %>