Add ONLY_SHOW_RELEVANT_REPOS back, fix explore page bug, make code more strict (#23766)

Follow #21962

After I eat my own dogfood, I would say that
ONLY_SHOW_RELEVANT_REPOS=false is necessary for many private/enterprise
instances, because many private repositories do not have
"description/topic", users just want to search by their names.

This PR also adds `PageIsExploreRepositories` check, to make code more
strict, because the `search` template is shared for different purpose.

And during the test, I found a bug that the "Search" button didn't
respect the "relevant" parameter, so this PR fixes the bug by the way
together.

I think this PR needs to be backported.

templates/explore/repo_search.tmpl

@@ -26,12 +26,15 @@
 	<input type="hidden" name="language" value="{{$.Language}}">
 	<div class="ui fluid action input">
 		<input name="q" value="{{.Keyword}}" placeholder="{{.locale.Tr "explore.search"}}…" autofocus>
+		{{if .PageIsExploreRepositories}}
+			<input type="hidden" name="only_show_relevant" value="{{.OnlyShowRelevant}}">
+		{{end}}
 		<button class="ui primary button">{{.locale.Tr "explore.search"}}</button>
 	</div>
 </form>
-{{if .OnlyShowRelevant}}
+{{if and .PageIsExploreRepositories .OnlyShowRelevant}}
 	<div class="ui message explore-relevancy-note">
-		<span data-tooltip-content="{{.locale.Tr "explore.relevant_repositories_tooltip"}}">{{.locale.Tr "explore.relevant_repositories" ((printf "%s%s" $.Link "?no_filter=1")|Escape) | Safe}}</span>
+		<span data-tooltip-content="{{.locale.Tr "explore.relevant_repositories_tooltip"}}">{{.locale.Tr "explore.relevant_repositories" ((printf "%s%s" $.Link "?only_show_relevant=0")|Escape) | Safe}}</span>
 	</div>
 {{end}}
 <div class="ui divider"></div>