Make logout_redirect_url configuration setting for Element Web configurable (#3903)

* Add logout_redirect_url var and option templating for matrix element client

* Simplify logout_redirect_url templating and make it safer

Using `to_json` to make it safer.

Judging by the code here https://github.com/element-hq/element-web/blob/0b24d33c64e5a161621f68fa8da65aaa8d1c847d/src/Lifecycle.ts#L1026-L1032
it doesn't seem like it leaving `logout_redirect_url` empty will hurt, so this patch removes the `if` block.

* Add comment for the `matrix_client_element_logout_redirect_url` variable

---------

Co-authored-by: Slavi Pantaleev <[email protected]>

Author: pviffx

roles/custom/matrix-client-element/defaults/main.yml

@@ -318,3 +318,7 @@ matrix_client_element_location_sharing_map_style_content_sources_localsource_til
 # matrix_client_element_location_sharing_map_style_content_sources_localsource_attribution: "&copy; <a href=\"https://www.openstreetmap.org/copyright\" target=\"_blank\">OpenStreetMap</a> contributors"
 # Leave blank, if map does not require attribution.
 matrix_client_element_location_sharing_map_style_content_sources_localsource_attribution: ""
+
+# Optional URL to redirect the user to after they have logged out.
+# See https://github.com/element-hq/element-web/blob/develop/docs/config.md#sso-setup
+matrix_client_element_logout_redirect_url: ''

roles/custom/matrix-client-element/templates/config.json.j2

@@ -40,6 +40,7 @@
 	{% if matrix_client_element_location_sharing_enabled %}
 	"map_style_url": {{ matrix_client_element_map_style_url | to_json }},
 	{% endif %}
+	"logout_redirect_url": {{ matrix_client_element_logout_redirect_url | to_json }},
 	"branding": {
 		"auth_footer_links": {{ matrix_client_element_branding_auth_footer_links | to_json }},
 		"auth_header_logo_url": {{ matrix_client_element_branding_auth_header_logo_url | to_json }},