Index overrides through `interpolate_index` do not work

This code seems to be incomplete as it is using the template directly without substitutions.

dashR/R/dash.R

Lines 1967 to 1969 in d281e70

	else if (length(private$template_index) == 1) {
	private$.index <- private$template_index
	}

Relatedly, index_string function also does not seem to be working because glue complains about missing title variable.

dashR/R/dash.R

Lines 1961 to 1965 in d281e70

	if (!is.null(private$custom_index)) {
	string_index <- glue::glue(private$custom_index, .open = "{%", .close = "%}")
	private$.index <- string_index
	}