feat(FastAPI): add /pkgbase/{name}/request (post)

This change implements the FastAPI version of the
/pkgbase/{name}/request form's action.

Changes from PHP:

- Additional errors are now displayed for the **merge_into** field,
  which are only displayed when the Merge type is selected.
    - If the **merge_into** field is empty, a new error is displayed:
      'The "Merge into" field must not be empty.'
    - If the **merge_into** field is given the name of a package base
      which does not exist, a new error is displayed:
      "The package base you want to merge into does not exist."
    - If the **merge_into** field is given the name of the package
      base that a request is being created for, a new error is
      displayed: "You cannot merge a package base into itself."
- When an error is encountered, users are now brought back to
  the request form which they submitted and an error is displayed
  at the top of the page.
- If an invalid type is provided, users are returned to a BAD_REQUEST
  status rendering of the request form.

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

templates/pkgbase/request.html

@@ -1,8 +1,17 @@
 {% 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>{{ "Submit Request" | tr }}: {{ pkgbase.Name }}</h2>
+
         <p>
             {{ "Use this form to file a request against package base "
             "%s%s%s which includes the following packages:"
@@ -15,7 +24,8 @@
         </ul>
 
         {# Request form #}
-        <form id="request-form" action="/pkgbase/{{ pkgbase.Name }}/request">
+        <form id="request-form" action="/pkgbase/{{ pkgbase.Name }}/request"
+              method="post">
             <fieldset>
                 <p>
                     <label for="id_type">{{ "Request type" | tr }}:</label>