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.

No files or tests are named. Start by tracing the existing event settings, registration-question, visa-letter, and travel-grant entry points to understand how these features are currently enabled. Done means event settings control the relevant registration questions and feature availability, with the configuration hidden for online events.

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