Since v0.26, Pomerium configures Envoy to use a custom HTML error page
format string for most errors served by Envoy itself. This format string
uses %COMMAND% directives to include details about the error.
The HTML error page template also includes any branding options set via
the corresponding Enterprise settings. We need to ensure that any %
signs in the branding options strings are escaped to %% so that Envoy
will not interpret them as the start of a %COMMAND% directive, which
could lead to Envoy rejecting the format string as invalid.
