Add ui.explore settings to control view of explore pages (2) (#14094)

This is an alternative PR to #13687.

Add `[ui.explore]` settings to allow restricting the
explore pages to logged in users only and to disable the users explore page.

The two proposed settings are:

- `REQUIRE_SIGNIN_VIEW`: Only allows access to the explore pages if the
user is signed in. Also restricts
  - `/api/v1/user/search`
  - `/api/v1/users/{username}`
  - `/api/v1/users/{username}/repos`
  - but does not restrict `/api/v1/users/{username}/heatmap`
- `DISABLE_USERS_PAGE`: Disables the /explore/users page

Fix #2908

Close #13687 

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: 6543 <6543@obermui.de>

routers/routes/web.go

@@ -286,6 +286,7 @@ func goGet(ctx *context.Context) {
 func RegisterRoutes(m *web.Route) {
 	reqSignIn := context.Toggle(&context.ToggleOptions{SignInRequired: true})
 	ignSignIn := context.Toggle(&context.ToggleOptions{SignInRequired: setting.Service.RequireSignInView})
+	ignExploreSignIn := context.Toggle(&context.ToggleOptions{SignInRequired: setting.Service.RequireSignInView || setting.Service.Explore.RequireSigninView})
 	ignSignInAndCsrf := context.Toggle(&context.ToggleOptions{DisableCSRF: true})
 	reqSignOut := context.Toggle(&context.ToggleOptions{SignOutRequired: true})
 
@@ -335,7 +336,7 @@ func RegisterRoutes(m *web.Route) {
 		m.Get("/users", routers.ExploreUsers)
 		m.Get("/organizations", routers.ExploreOrganizations)
 		m.Get("/code", routers.ExploreCode)
-	}, ignSignIn)
+	}, ignExploreSignIn)
 	m.Get("/issues", reqSignIn, user.Issues)
 	m.Get("/pulls", reqSignIn, user.Pulls)
 	m.Get("/milestones", reqSignIn, reqMilestonesDashboardPageEnabled, user.Milestones)