Fix url leak in application templates guide

Encapsulate url that was including the trailing quote and colon.
Merge pull request #13338 from shreve/patch-1

Fix url leak in application templates guide
Remove the link for code.whytheluckystiff.net

This is not a valid URL.

[ci skip]
Default I18n.enforce_available_locales to true

We will default this option to true from now on to ensure users properly
handle their list of available locales whenever necessary. This option
was added as a security measure and thus Rails will follow it defaulting
to secure option.

Also improve the handling of I18n config options in its railtie, taking
the new enforce_available_locales option into account, by setting it as
the last one in the process. This ensures no other configuration will
trigger a deprecation warning due to that setting.
Disable available locales checks to avoid warnings running the tests
Add changelog and upgrading notice related to I18n enforce_available_locales handling