Use a placeholder for "Terms & Conditions" in translatable strings

There are several places the phrases "Terms & Conditions" and "T&C" are used. I'm maintaining a German website and had to change all messages to reflect my own terminology. Placeholders should be introduced to avoid double translations and enhance consistency, because there are several different names for T&Cs in German.

I'm not sure, if also for English websites "Terms & Conditions" should be customizable. Likely in English there are also many different names. Some might want to call it General Terms and Conditions or reduce it to Terms. They would have to change the code to achieve that.

The strings are:

- Terms & Conditions
- No Terms & Conditions have been added yet.
- No users have accepted Terms & Conditions yet.
- <strong>Accept</strong> Terms & Conditions of Use
- Your Terms & Conditions
- Terms & Conditions have been saved.
- Terms & Conditions will not be shown to users, as no T&C have been saved.
- To continue to use this site please read the Terms & Conditions below, and complete the form to confirm your acceptance.
- Changes to the Terms & Conditions since last accepted:
- <strong>Accept</strong> <a href="@terms">Terms & Conditions</a> of Use
- Display a Terms & Conditions statement on the registration page, require visitor to accept T&C to register. When a user creates an account they are required to accept your Terms & Conditions to complete their registration.