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 modifies FIELD_TYPE_DATA dict in `CustomProfileField`
model to store keyword of field types. And create new dict
FIELD_TYPE_CHOICES_DICT to store all field type information
by field type keyword, i.e. id, name.
This is preparatory commit to remove all static use of field
types in frontend and access field type with keyword instead
of display name.
Originally was going to centralize this in zerver/lib/request.pyi, but this
file is not visible at run-time, being only a stub. The matching request.py
file seemed inappropriate, as it doesn't actually use ViewFuncT.
