Consolidate back-end logic with corresponding front-end code as plugin

[travelist]

For future contributor, it's likely to say that the changes they have to do to add new functionality should not be widely. For reducing the change areas, we would like to support the plugin which is the set of backend logic with UI part. This allows contributors to easily add new component as a plugin.

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Need to discuss:

1. the location of plugin directory which can compose Javascript, html, css, and python logic.
2. Sometimes new component would like to use Database, so how to support.
3. how to load plugins

this is not all, when you have ideas which I'm not aware of, please pose here.