Adapt Smarty templates to less forgiving PHP8 environment

[asmecher]

PHP8 is pickier about code in ways that affect Smarty templates. In order to enable PHP8 support, it was necessary to extend Smarty's error suppression so that new warnings didn't cause problems:

3c88c04

See the Smarty discussion here:

smarty-php/smarty#605

It appears that the best long-term solution will be to adapt our Smarty templates so that they do not use undefined variables. This will be a big task as Smarty was previously forgiving about this and we frequently made use of it.

In order to do this, on a machine running PHP8...

• Revert the change to PKPTemplateManager that suppresses warnings
• Work through the warnings that result.

(Edit: We will need to think through the implications for third parties of making Smarty significantly stricter. One possible solution will be to introduce a "strict" mode, so our codebase can be coded as tightly as possible, but have it permit some flexibility when that strict mode is disabled.)