Initial support for LDAP server authentication

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

package.json

@@ -85,6 +85,7 @@
     "passport-github": "^1.1.0",
     "passport-gitlab2": "^2.2.0",
     "passport-google-oauth20": "^1.0.0",
+    "passport-ldapauth": "^0.6.0",
     "passport-local": "^1.0.0",
     "passport-twitter": "^1.0.4",
     "passport.socketio": "^3.6.2",