Use a macro for form tags

This will stop us repeatedly forgetting to add `novalidate` and
`autocomplete='off'` to our forms (which is how most of them are set
up).

It uses sensible defaults, based on how we most-commonly configure
forms:
- most of our forms are `post`ed (but this can be overridden)
- `autocomplete` should only be enabled where it makes sense, otherwise
  it’s more annoying than useful (but this can be overriden)
- we should never be using HTML5 form validation because our own error
  styles and messages are better

app/templates/components/form.html

@@ -0,0 +1,20 @@
+{% macro form_wrapper(
+  method="post",
+  action=None,
+  autocomplete=False,
+  class=None,
+  id=None,
+  module=None
+) %}
+  <form
+    method="{{ method }}"
+    {% if action %}action="{{ action }}"{% endif %}
+    {% if not autocomplete %}autocomplete="off"{% endif %}
+    {% if class %}class="{{ class }}"{% endif %}
+    {% if id %}id="{{ id }}"{% endif %}
+    {% if module %}data-module="{{ module }}"{% endif %}
+    novalidate
+  >
+    {{ caller() }}
+  </form>
+{% endmacro %}