[feature] Add a request ID and include it in logs (#1476)

This adds a lightweight form of tracing to GTS. Each incoming request is
assigned a Request ID which we then pass on and log in all our log
lines. Any function that gets called downstream from an HTTP handler
should now emit a requestID=value pair whenever it logs something.

Co-authored-by: kim <grufwub@gmail.com>

web/template/404.tmpl

@@ -29,7 +29,8 @@
 		</p>
 		<p>
 			If you believe this 404 was an error, you can contact
-			the instance admin.
+			the instance admin. Provide them with the following request
+			Request ID: <code>{{.requestID}}</code>.
 		</p>
 	</section>
 </main>

web/template/error.tmpl

@@ -20,6 +20,7 @@
     <main>
         <section class="error">
           <span>❌</span> <pre>{{.error}}</pre>
+          <span>Request ID</span> <code>{{.requestID}}</code>
         </section>
     </main>
 {{ template "footer.tmpl" .}}