feat(fastapi): add /pkgbase/{name}/merge (post)

Changes:

- `via` is not required in FastAPI. We deduce the involved
  requests via their PackageBaseName / MergeBaseName columns
  and set them to Accepted when merged.
- When erroneous input is given, the error is now presented
  on the merge page instead of sending the user to the pkgbase
  page.

Signed-off-by: Kevin Morris <kevr@0cost.org>

templates/pkgbase/merge.html

@@ -1,6 +1,15 @@
 {% extends "partials/layout.html" %}
 
 {% block pageContent %}
+
+    {% if errors %}
+        <ul class="errorlist">
+            {% for error in errors %}
+                <li>{{ error | tr }}</li>
+            {% endfor %}
+        </ul>
+    {% endif %}
+
     <div class="box">
         <h2>{{ "Merge Package" | tr }}: {{ pkgbase.Name }}</h2>