Don't warn users about *every* dirty form (#3707)

The choice regarding which forms should or should not trigger a warning
is subjective. I tried to be consistent and not warn about forms that:
 - run an action, rather than edit data: search, send an email.
 - delete data: a warning about losing data would be confusing

Note that forms on sign-in pages were already ignored (using a selector,
rather than an explicit class on the form element).

Fixes #3698.

templates/admin/base/search.tmpl

@@ -17,7 +17,7 @@
 		</div>
 	</div>
 </div>
-<form class="ui form"  style="max-width: 90%">
+<form class="ui form ignore-dirty"  style="max-width: 90%">
 	<div class="ui fluid action input">
 	  <input name="q" value="{{.Keyword}}" placeholder="{{.i18n.Tr "explore.search"}}..." autofocus>
 	  <button class="ui blue button">{{.i18n.Tr "explore.search"}}</button>