registration: Enable import from slack using realm registration form.

Co-authored-by: Alex Vandiver <alexmv@zulip.com>
Co-authored-by: Tim Abbott <tabbott@zulip.com>

tools/linter_lib/custom_check.py

@@ -569,6 +569,7 @@ html_rules: list["Rule"] = [
         "description": "`placeholder` value should be translatable.",
         "exclude_line": {
             ("templates/zerver/realm_creation_form.html", 'placeholder="acme"'),
+            ("templates/zerver/slack_import.html", 'placeholder="xoxb-…"'),
         },
         "exclude": {
             "templates/corporate",