Add new custom profile field type, External account.
External account field links user's social media
profile with account. e.g. GitHub, Twitter, etc.
Fixes part of #12302
This commit add FIELD_TYPE_CHOICES_DICT to page_params and replace
FIELD_TYPE_CHOICES.
FIELD_TYPE_CHOICES_DICT includes all field types with keyword, id
and display name. Using this field-type-dict, we can access field
type information by it's keyword, and remove all static use of
field-type'a name or id in frontend.
This commit also modifies functions in js where this page_params
field-types is used.
Hide `delete-btn` of first choice in create-new-custom-field and
edit-choice-type-field form, so user can not delete all choices
and have to submit at least one choice.
This commit remove this logic of choice-field-delete-btn from
both settings.
This is preparatory commit of adding new logic in `delete-btn`.
Anders Kaseorg
Yashashvi Dave
Steve Howell