[Feature Request]: Customizable Gravatar support

[tb0hdan]

Description

Hi,

Canadian users can now receive emails, thanks to a gateway running on mt.gt domain.

Node mapping works as follows:
! -> 0x, i.e message for 0xdeadbeef@mt.gt gets sent to !deadbeef node.

Having a drop-down in settings, or a custom text field for user to be able to enter mt.gt will allow the Web UI to call Gravatar API and fetch icons.

Link to same issue for Android: meshtastic/Meshtastic-Android#1127

Thanks.