Add download zonefile button to external DNS page (#1853)

Co-authored-by: Joshua Tauberer <jt@occams.info>

api/mailinabox.yml

@@ -743,6 +743,31 @@ paths:
             text/html:
               schema:
                 type: string
+  /dns/zonefile/{zone}:
+      get:
+          tags:
+              - DNS
+          summary: Get DNS zonefile
+          description: Returns an array of all managed top-level domains.
+          operationId: getDnsZonefile
+          x-codeSamples:
+              - lang: curl
+                source: |
+                    curl -X GET "https://{host}/admin/dns/zonefile/<zone>" \
+                      -u "<email>:<password>"
+          responses:
+              200:
+                  description: Successful operation
+                  content:
+                      application/json:
+                          schema:
+                              $ref: '#/components/schemas/DNSZonefileResponse'
+              403:
+                  description: Forbidden
+                  content:
+                      text/html:
+                          schema:
+                              type: string
   /dns/update:
     post:
       tags:
@@ -2050,6 +2075,8 @@ components:
       items:
         $ref: '#/components/schemas/Hostname'
       description: DNS zones response.
+    DNSZonefileResponse:
+      type: string
     DNSSecondaryNameserverResponse:
       type: object
       required: