Keep what was typed when a target or webhook edit is refused (closes #381)

A refused save on the target edit page now shows the edit form again,
with the reason above it and every value submitted, instead of a bare
text page; the status codes are unchanged. The webhook edit page keeps
the submitted name, description and retention the same way, while the
page still reports the stored retention.

Target edits are validated by setTargetFromForm, which newTarget now
uses too, so the add and edit forms accept and refuse the same things.
An empty max_retries keeps the target's own count.

The browser test also saves both edit pages with refused values.

Model: opus-5-5

templates/source_edit.html

@@ -18,17 +18,17 @@
             <input type="hidden" name="csrf_token" value="{{.CSRFToken}}">
             <div class="form-group">
                 <label for="name" class="label">Name</label>
-                <input type="text" id="name" name="name" value="{{.Webhook.Name}}" required class="input">
+                <input type="text" id="name" name="name" value="{{.Name}}" required class="input">
             </div>
 
             <div class="form-group">
                 <label for="description" class="label">Description</label>
-                <textarea id="description" name="description" rows="3" class="input">{{.Webhook.Description}}</textarea>
+                <textarea id="description" name="description" rows="3" class="input">{{.Description}}</textarea>
             </div>
 
             <div class="form-group">
                 <label for="retention_days" class="label">Retention (days)</label>
-                <input type="number" id="retention_days" name="retention_days" value="{{.Webhook.RetentionDays}}" min="0" class="input">
+                <input type="number" id="retention_days" name="retention_days" value="{{.RetentionDays}}" min="0" class="input">
                 <p class="text-xs text-gray-500 mt-1">Currently {{.Webhook.RetentionLabel}}.{{if .Webhook.RetainsForever}} No events are deleted while retention is set to forever.{{else}} A periodic cleanup permanently deletes events older than this, along with their delivery records.{{end}} Enter 0 to retain events forever; leave blank to keep the current setting.</p>
             </div>