Check if file exists before deleting in destroy()

Added file_exists check before deleting session file in destroy() of VuFind/Session/File.php to prevent Warning: unlink(): No such file or directory
Created separate class for determining configuration location (vs. reading).
Use view helper instead of direct config access.
Changed self:: to static:: for greater extensibility.
Eliminated direct use of \VuFind\Config\Reader.
Eliminated direct use of \VuFind\Config\Reader.
Inject configuration into Summon connection.
Inject WorldCat configuration.
Inject configuration into WorldCatUtils.
Inject configuration.
Inject configuration.
Inject configuration.
Cleaned up use statements.
Whitespace cleanup.
HMAC-related dependency injection.
Removed unnecessary use statement.
Dependency injection for database adapter factory.
Fixed bug in unit test.
Style fix.
Removed unnecessary use statement.