* 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 0b24d33c64/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

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Co-authored-by: Slavi Pantaleev <slavi@devture.com>