Plugin API improvements

[WithoutPants]

These changes are intended to improve the external interface provided for plugins. These interfaces are meant for custom plugin UIs to call.

• Makes the task_name parameter in runPluginTask optional. If no task_name is provided, then the plugin interface is executed without any additional arguments that might be provided by a task config. This method of running a plugin task is intended for plugins that want to run tasks using the task queue but don't want to expose the task in the stock UI.
• Deprecates args in runPluginTask in favour of the more generic args_map map input.
• Adds a description parameter in runPluginTask, which will be used in the task queue description, which is otherwise defaulted to the description in the task config if task_name is provided, or the plugin ID otherwise.
• Adds a new runPluginOperation which runs a plugin with the supplied arguments immediately, not using the task management system, and returns the result provided by the Output field of the plugin. Returns an error if the Error field is populated.