This determines which ldap field is used as the username on
HackMD. By default, the "id" is used as username, too. The id
is taken from the fields `uidNumber`, `uid` or
`sAMAccountName`. To give the user more flexibility, they can
now choose the field used for the username instead.
Limitations as of this commit:
- tlsOptions can only be specified in config.json, not as env vars
- authentication failures are not yet gracefully handled by the UI
- instead the error message is shown on a blank page (/auth/ldap)
- no email address is associated with the LDAP user's account
- no picture/profile URL is associated with the LDAP user's account
- we might have to generate our own access + refresh tokens,
because we aren't using oauth. The currently generated
tokens are just a placeholder.
- 'LDAP Sign in' needs to be translated to each locale
