refactor api package comments

* use encoding/json for comment parsing

* refactor by moving loop code to an UnmarshalJSON

* use a preallocated array and indices to maintain order while using
  goroutines again, this was removed a while ago

* use new struct in comment.hbs and contextComment.hbs

* rewriteUrl partial to reduce rimgo-specific code in api

* move RenderError into pages package to avoid import cycle between render and utils

utils/rewriteUrl.go

@@ -0,0 +1,25 @@
+package utils
+
+import (
+	"fmt"
+	"net/url"
+	"strings"
+)
+
+func RewriteUrl(link string) (string, error) {
+	url, err := url.Parse(link)
+	if err != nil {
+		return "", err
+	}
+	path := url.Path
+	if !strings.HasPrefix(path, "/") {
+		path = "/" + path
+	}
+	switch url.Host {
+	case "", "imgur.com", "www.imgur.com", "i.imgur.com":
+		return path, nil
+	case "i.stack.imgur.com":
+		return "/stack" + path, nil
+	}
+	return "", fmt.Errorf("unknown host %s", url.Host)
+}