add edit thread routes

app/templates/threads/edit.html

@@ -0,0 +1,11 @@
+{%- from 'common/macros.html' import subheader, babycode_editor_component -%}
+{%- extends 'base.html' -%}
+{%- block title -%}editing thread "{{thread.title}}"{%- endblock -%}
+{%- block content -%}
+{{subheader('Rename thread "%s"' % thread.title, 'You can change the thread title here. To edit the OP of this thread, press the Edit button on the post instead.')}}
+<form class="plank primary-bg full-width" method="POST">
+  <label for="title">New title</label>
+  <input type="text" id="title" name="title" required value="{{thread.title}}">
+  <input type="submit" value="Save">
+</form>
+{%- endblock -%}