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

public/views/signin-modal.ejs

@@ -38,7 +38,13 @@
                     <i class="fa fa-google"></i> <%= __('Sign in via %s', 'Google') %>
                 </a>
                 <% } %>
-                <% if((facebook || twitter || github || gitlab || dropbox || google) && email) { %>
+                <% if(ldap) { %>
+                <a type="button" class="btn btn-lg btn-block btn-social btn-reddit "data-toggle="modal" data-target=".signin-ldap-modal">
+                    <i class="fa fa-book"></i> <%= __('Sign in via %s', 'LDAP') %>
+                </a>
+                <% } %>
+
+                <% if((facebook || twitter || github || gitlab || dropbox || google || ldap) && email) { %>
                 <hr>
                 <% }%>
                 <% if(email) { %>
@@ -67,4 +73,4 @@
             </div>
         </div>
     </div>
-</div>
+</div>