[BUG] Remember topic only in repo search

- Backport of #2489
- If the user is searching repositories with an specific topic, adding
any other filter option, such as showing unrelevant repositories or
using another sort Forgejo should remember that 'topic only' was set.
- Adds integration test.
- Resolves #2461

(cherry picked from commit b4360d504c)

templates/explore/repo_search.tmpl

@@ -2,6 +2,7 @@
 	<form class="ui form ignore-dirty gt-f1">
 		<input type="hidden" name="sort" value="{{$.SortType}}">
 		<input type="hidden" name="language" value="{{$.Language}}">
+		<input type="hidden" name="topic" value="{{$.TopicOnly}}">
 		<div class="ui fluid action input">
 			{{template "shared/searchinput" dict "Value" .Keyword "AutoFocus" true}}
 			{{if .PageIsExploreRepositories}}

tests/integration/explore_repos_test.go

@@ -8,6 +8,8 @@ import (
 	"testing"
 
 	"code.gitea.io/gitea/tests"
+
+	"github.com/stretchr/testify/assert"
 )
 
 func TestExploreRepos(t *testing.T) {
@@ -15,4 +17,16 @@ func TestExploreRepos(t *testing.T) {
 
 	req := NewRequest(t, "GET", "/explore/repos")
 	MakeRequest(t, req, http.StatusOK)
+
+	t.Run("Persistent parameters", func(t *testing.T) {
+		defer tests.PrintCurrentTest(t)()
+
+		req := NewRequest(t, "GET", "/explore/repos?topic=1&language=Go&sort=moststars")
+		resp := MakeRequest(t, req, http.StatusOK)
+		htmlDoc := NewHTMLParser(t, resp.Body)
+
+		assert.EqualValues(t, "moststars", htmlDoc.Find("input[type='hidden'][name='sort']").AttrOr("value", "not found"))
+		assert.EqualValues(t, "Go", htmlDoc.Find("input[type='hidden'][name='language']").AttrOr("value", "not found"))
+		assert.EqualValues(t, "true", htmlDoc.Find("input[type='hidden'][name='topic']").AttrOr("value", "not found"))
+	})
 }