webhooks: add "user created" webhook

Webhooks send a POST to an admin-supplied URL when something happens,
with relevant information sent in JSON. One has been added for creating
users in Settings > Webhooks > User Created.

Lazily, the portion of GetUsers which generates a respUser has been
factored out, and is called to send the JSON payload.

A stripped-down common.Req method has been added, which is used by the
barebones WebhookSender struct.

common/common.go

@@ -1,10 +1,16 @@
 package common
 
 import (
+	"bytes"
+	"compress/gzip"
+	"encoding/json"
 	"errors"
 	"fmt"
+	"io"
 	"log"
 	"net/http"
+	"net/url"
+	"strings"
 
 	lm "github.com/hrfee/jfa-go/logmessages"
 )
@@ -77,3 +83,68 @@ type ConfigurableTransport interface {
 	// SetTransport sets the http.Transport to use for requests. Can be used to set a proxy.
 	SetTransport(t *http.Transport)
 }
+
+// Stripped down-ish version of rough http request function used in most of the API clients.
+func Req(httpClient *http.Client, timeoutHandler TimeoutHandler, mode string, uri string, data any, queryParams url.Values, headers map[string]string, response bool) (string, int, error) {
+	var params []byte
+	if data != nil {
+		params, _ = json.Marshal(data)
+	}
+	if qp := queryParams.Encode(); qp != "" {
+		uri += "?" + qp
+	}
+	var req *http.Request
+	if data != nil {
+		req, _ = http.NewRequest(mode, uri, bytes.NewBuffer(params))
+	} else {
+		req, _ = http.NewRequest(mode, uri, nil)
+	}
+	req.Header.Add("Content-Type", "application/json")
+	for name, value := range headers {
+		req.Header.Add(name, value)
+	}
+	resp, err := httpClient.Do(req)
+	if resp == nil {
+		return "", 0, err
+	}
+	err = GenericErr(resp.StatusCode, err)
+	if timeoutHandler != nil {
+		defer timeoutHandler()
+	}
+	var responseText string
+	defer resp.Body.Close()
+	if response || err != nil {
+		responseText, err = decodeResp(resp)
+		if err != nil {
+			return responseText, resp.StatusCode, err
+		}
+	}
+	if err != nil {
+		var msg any
+		err = json.Unmarshal([]byte(responseText), &msg)
+		if err != nil {
+			return responseText, resp.StatusCode, err
+		}
+		if msg != nil {
+			err = fmt.Errorf("got %d: %+v", resp.StatusCode, msg)
+		}
+		return responseText, resp.StatusCode, err
+	}
+	return responseText, resp.StatusCode, err
+}
+
+func decodeResp(resp *http.Response) (string, error) {
+	var out io.Reader
+	switch resp.Header.Get("Content-Encoding") {
+	case "gzip":
+		out, _ = gzip.NewReader(resp.Body)
+	default:
+		out = resp.Body
+	}
+	buf := new(strings.Builder)
+	_, err := io.Copy(buf, out)
+	if err != nil {
+		return "", err
+	}
+	return buf.String(), nil
+}