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

lib/response.js

@@ -66,6 +66,7 @@ function showIndex(req, res, next) {
         gitlab: config.gitlab,
         dropbox: config.dropbox,
         google: config.google,
+        ldap: config.ldap,
         email: config.email,
         signin: req.isAuthenticated(),
         infoMessage: req.flash('info'),
@@ -98,6 +99,7 @@ function responseHackMD(res, note) {
         gitlab: config.gitlab,
         dropbox: config.dropbox,
         google: config.google,
+        ldap: config.ldap,
         email: config.email
     });
 }