Add a form to edit settings

Very basic form without style, just to allow to edit some settings and see the result
You could use a simpler syntax:
```coffeescript
settings =
    messagesPerPage = SettingsStore.get 'messagesPerPage'
    composeInHTML   = SettingsStore.get 'componseInHTML'
```
Or even simpler:
```coffeescript
settings = SettingsStore.get()
```