websites/d…and…t: Configure formsubmit

To handle the RSVP form on *dustinandtabitha.com*, we are going to use
*formsubmit*.  It runs on the same machine that hosts the website, so
there's no dealing with CORS.  The */submit/rsvp* path, which is proxied
to the backend, is the RSVP form's target.

roles/websites/dustinandtabitha.com/vars/main.yml

@@ -0,0 +1,10 @@
+dustinandtabitha_formsubmit:
+  csvfile: /srv/www/dustinandtabitha/data/rsvp.csv
+  defaults:
+    adult: 0
+    preschool: 0
+    school: 0
+  matrix:
+    homeserver: hatch.chat
+    room: '!qJoqJkHxCHgDSTZwwX:hatch.chat'
+    access_token: '{{ dustinandtabitha_formsubmit_matrix_token }}'