zephyr_subscribe_to refactoring

* Split to `subscribe_to_tzc` and `subscribe_to_zeph02`
* Add member function `subscribe_to` to `zephyr_account` to reduce comparisons
* Remove unused `galaxy` parameter
* Convert `while` to `for` around one of `subscribe_to` calls

Testing Done:
Compile only.

Reviewed at https://reviews.imfreedom.org/r/422/