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

locales/en.json

@@ -100,5 +100,6 @@
 	"Select From Available Snippets": "Select From Available Snippets",
 	"OR": "OR",
 	"Export to Snippet": "Export to Snippet",
-	"Select Visibility Level": "Select Visibility Level"
+	"Select Visibility Level": "Select Visibility Level",
+	"LDAP Sign in": "LDAP Sign in"
 }