Step 2 : Allow addition of custom fields in the attendee-form

[uds5501]

Parent Issue : #2956

This issue is focused on implementing a basic custom question addition in the attendee forms just like event-brite.

Proposed addition in the custom-fields model:

• Question Prompt : To let organisers add details about the custom question
• IsCustomQuestion : To separate the default fields from these custom fields.

[prateekj117]

@uds5501 May I work on it's server side implementation?

[uds5501]

@prateekj117 thanks for the help but I have already prepared both server and backend PRs. I will be pushing both of them tonight.