Hard deprecate show_rcode_modal and remove `inst/js/init.js`

[m7pr]

After the release of teal 0.15.3 please hard deprecate show_rcode_modal. inst/js/init.js should be deleted as it contains javascript code used only in show_rcode_modal

[gogonzo]

init.js contains a code which is used in show R code modal only. When r code modal will be removed from teal it should be removed as well

See occurrences of data-clipboard-target in the whole organization

[llrs-roche]

I was checking other things and realize that data-clipboard-target is only used in teal. Maybe is time to deprecate it in favor of teal.widgets::verbatim_popup, but that function is marked as experimental