Index overrides through `interpolate_index` do not work

[ghost]

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

https://github.com/plotly/dashR/blob/d281e7025d0906f6f65e0365d25d2a59c362ed9e/R/dash.R#L1967-L1969

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

https://github.com/plotly/dashR/blob/d281e7025d0906f6f65e0365d25d2a59c362ed9e/R/dash.R#L1961-L1965