Feature/php8 support (#629)

Adds support for PHP8.0, dropping support for PHP7.0 and below.

Backwards incompatible changes:
- Dropped support for php asp tags in templates (removed from php since php7.0)
- Dropped deprecated API calls that where only accessible through SmartyBC
- Dropped support for {php} and {include_php} tags and embedded PHP in templates. Embedded PHP will now be passed through as is.
- Removed all PHP_VERSION_ID and compare_version checks and conditional code blocks that are now no longer required
- Dropped deprecated SMARTY_RESOURCE_CHAR_SET and SMARTY_RESOURCE_DATE_FORMAT constants
- Dropped deprecated Smarty::muteExpectedErrors and Smarty::unmuteExpectedErrors API methods
- Dropped deprecated $smarty->getVariable() method. Use $smarty->getTemplateVars() instead.
- $smarty->registerResource() no longer accepts an array of callback functions

See the changelog for more details.

Switched CI from Travis to Github CI.

libs/sysplugins/smarty_internal_method_clearcompiledtemplate.php

@@ -22,7 +22,7 @@ class Smarty_Internal_Method_ClearCompiledTemplate
      * Delete compiled template file
      *
      * @api  Smarty::clearCompiledTemplate()
-     * @link http://www.smarty.net/docs/en/api.clear.compiled.template.tpl
+     * @link https://www.smarty.net/docs/en/api.clear.compiled.template.tpl
      *
      * @param \Smarty $smarty
      * @param string  $resource_name template name