Fix parsing the LDAP_CONTACTS environment variable

The current code skips loading contact information from LDAP if
`!process.env.LDAP_CONTACTS` evaluates to `true`.

This is nearly never the case, as `process.env` contains strings
and non-empty strings evaluate to `true`, making the negation falsy.
Only an empty string in `LDAP_CONTACTS` (or not setting the environment
variable at all) skips the contact loading.

This commit changes the logic to only load contacts from LDAP if the
`LDAP_CONTACTS` environment variable is explicitly set to `"true"` (case
insensitive). This should bring the behaviour of the application more in
line with the expectation and the docs.