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/repo/commits_table.tmpl

@@ -5,7 +5,7 @@
 		</div>
 		<div class="ten wide right aligned column">
 			{{if .PageIsCommits}}
-				<form action="{{.RepoLink}}/commits/{{.BranchNameSubURL}}/search">
+				<form class="ignore-dirty" action="{{.RepoLink}}/commits/{{.BranchNameSubURL}}/search">
 					<div class="ui tiny search input">
 						<input name="q" placeholder="{{.i18n.Tr "repo.commits.search"}}" value="{{.Keyword}}" autofocus>
 					</div>

templates/repo/home.tmpl

@@ -10,7 +10,7 @@
 			</div>
 			{{if .RepoSearchEnabled}}
 				<div class="ui repo-search">
-					<form class="ui form" action="{{.RepoLink}}/search" method="get">
+					<form class="ui form ignore-dirty" action="{{.RepoLink}}/search" method="get">
 						<div class="field">
 							<div class="ui action input">
 								<input name="q" value="{{.Keyword}}" placeholder="{{.i18n.Tr "repo.search.search_repo"}}">

templates/repo/issue/search.tmpl

@@ -1,4 +1,4 @@
-<form class="ui form">
+<form class="ui form ignore-dirty">
 	<div class="ui fluid action input">
 		<input type="hidden" name="type" value="{{$.ViewType}}"/>
 		<input type="hidden" name="state" value="{{$.State}}"/>

templates/repo/search.tmpl

@@ -3,7 +3,7 @@
 	{{template "repo/header" .}}
 	<div class="ui container">
 		<div class="ui repo-search">
-			<form class="ui form" method="get">
+			<form class="ui form ignore-dirty" method="get">
 				<div class="ui fluid action input">
 					<input name="q" value="{{.Keyword}}" placeholder="{{.i18n.Tr "repo.search.search_repo"}}">
 					<button class="ui button" type="submit">