Fix missing language string error
Fix missing language string error

When viewing an issue's Related Changesets (in view.php), an error
message 'String "plugin_SourceIntegration_affected_issues" not found.'
is displayed.

This is because the plugin context in View Issue page is not 'Source',
but 'SourceIntegration', so the plugin_lang_get() call needs to
reference the base plugin.

Regression introduced by 5aec11023a9c242a67f2255e24b4377d28c6a4f4 (#224)

Fixes #231

dregad/source-integration,make-all/source-integration,dregad/source-integration,mantisbt-plugins/source-integration,mantisbt-plugins/source-integration,dregad/source-integration,mantisbt-plugins/source-integration,make-all/source-integration,make-all/source-integration