Improve invalidation of stored service password if the endpoint URL is changed

A number of issues were found in !1490 and !1558 (triggered by support request 7395)

* It is not possible to set a new URL and a password at the same time (new password is ignored)
* An error occurs on the Service Templates admin pages (prop_updated? was referencing the service's project, which is not defined for templates)
* Passwords are reset on every save in Service Templates admin pages

This should fix these 3 issues by respectively:
* Differentiating a property that has been assigned a new value (regardless of the new value) and a property that has been assigned a new value that is different from the old one
* Providing an alternate implementation to detected updated properties, not relying on the service's project
* Filtering an empty password parameter passed to the Service Templates admin page like on the project service page

See merge request !1583