Cannot add sapling address in Web UI

Keybase client lets me add sapling addresses with `$ keybase currency add zs...`. The website still says `You can post either a z_address or a t_address. Sapling coming next release cycle` and pasting a sapling address does not light up the continue button.

Start by reproducing the website currency-address flow with a sapling address and compare it with the existing z_address and t_address behavior. Done means a pasted sapling address enables the Continue button and the outdated message no longer says Sapling is coming next release cycle.

Written by the indexing model from the issue text.