v0.8.19

WebIO v0.8.19

Diff since v0.8.18

Closed issues:

• setexcludinghandlers! was removed in Observables 0.5 (#493)
• Tests fail locally (#499)

Merged pull requests:

• Update use of Observables listeners to current interface (#497) (@halleysfifthinc)
• Fix frontend test (#501) (@olegshtch)
• Set minimum Julia version to 1.6 (#502) (@fonsp)