Migrate Theme Options/Options Framework to Customizer controls

Past research on this topic:

This will encompass:

[ ] research how the Customizer prefers to deal with data
[ ] losslessly morph the existing optionsframework metadata to theme mods format as part of Largo's upgrade function
[ ] test for that upgrade
[ ] provide shim function for of_get_option and of_set_option if possible for child themes to prevent immediately-upon-upgrade breakage
[ ] recreate the Theme Options settings in the Customizer, possibly using "selective refresh" as described in https://github.com/INN/largo/issues/1438
[ ] replace Largo's custom site icon logic with WordPress' in https://github.com/INN/largo/issues/835
[ ] check if https://github.com/INN/largo/issues/243 is still an issue
[ ] (low priority) fix https://github.com/INN/largo/issues/1823

WPBuddy / largo