Travel & Accommodation configuration restructure

Currently the registration questions related to travel & accommodation are regular registration questions which an organiser can enable / disable as they please. This should rather be configurable in the event settings which would cause the relevant questions to be added to the registration form.

For online events, this configuration should not be visible at all.

TODO's:

Add configuration options for the following:
- Do you provide travel grants to participants? (yes/no)
- Do you provide free accommodation to participants? (yes/no)
- Do you want to enable automatic visa invitation letter generation? (yes/no)

If any one of these are enabled, show a Travel section in the registration form, with the relevant questions from the list below:

1) Do you want to apply for a travel grant? (yes/no)
1b) If yes, ask Country of Travel & City of Travel
2) Do you want to apply for free accommodation? (yes/no)

Also we should enable/disable the corresponding features for Visa letter generation and travel grant submissions based on the above configuration.

ファイルやテストは指定されていません。まず、既存のイベント設定、登録時の質問、ビザレター、旅行助成金のエントリーポイントを追跡し、これらの機能が現在どのように有効化されているかを把握してください。イベント設定によって関連する登録時の質問と機能の利用可否が制御され、オンラインイベントでは設定が非表示になれば完了です。

索引モデルが issue の本文から書いたものです。