Fix plugin site option getter/setter code path

Apparently the plugin site_conf_form has never been used
as the underlying code to get and set the options was
broken/missing.
Merge pull request #1311 from jaysoffian/fix_plugin_site_options

Fix plugin site option getter/setter code path
Merge pull request #1310 from jaysoffian/master

Add Project.delete_option() and fix bug in ProjectOption.unset_value()
pylint: remove unused import