MM-10370: serve subpath (#8968)

* factor out GetSubpathFromConfig

* mv web/subpath.go to utils/subpath.go

* serve up web, api and ws on /subpath if configured

* pass config to utils.RenderWeb(App)?Error

This allows the methods to extract the configured subpath and redirect
to the appropriate `/subpath/error` handler.

* ensure GetSubpathFromConfig returns trailing slashes deterministically

* fix error 404 handling

* redirect /subpath to /subpath/

This is necessary for the static handler to match, otherwise none of the
registered routes find anything. This also makes it no longer necessary
to add trailing slashes in the root router.

api4/file.go

@@ -312,13 +312,13 @@ func getPublicFile(c *Context, w http.ResponseWriter, r *http.Request) {
 
 	if len(hash) == 0 {
 		c.Err = model.NewAppError("getPublicFile", "api.file.get_file.public_invalid.app_error", nil, "", http.StatusBadRequest)
-		utils.RenderWebAppError(w, r, c.Err, c.App.AsymmetricSigningKey())
+		utils.RenderWebAppError(c.App.Config(), w, r, c.Err, c.App.AsymmetricSigningKey())
 		return
 	}
 
 	if hash != app.GeneratePublicLinkHash(info.Id, *c.App.Config().FileSettings.PublicLinkSalt) {
 		c.Err = model.NewAppError("getPublicFile", "api.file.get_file.public_invalid.app_error", nil, "", http.StatusBadRequest)
-		utils.RenderWebAppError(w, r, c.Err, c.App.AsymmetricSigningKey())
+		utils.RenderWebAppError(c.App.Config(), w, r, c.Err, c.App.AsymmetricSigningKey())
 		return
 	}