syncthing: update to 0.12.15

[awiouy]

Hello,

Here are updates to the syncthing addon, mainly:

• enable setting of the GUI address,
• skip default folder creation on new install, and
• upgrade to version 0.12.15.

I expect that this should hold for a while, in particular since syncthing provides for automatic upgrades.

Thank you in advance, and regards, @

[lrusak]

Thanks again