Expose info endpoint via proxy

[kilrau]

xud-ui should be able to judge wether

• password is still default or was already set by user
• mnemonic already shown or not
• backup directory still default or not

[reliveyy]

The response of endpoint /api/v1/info will be JSON like @krrprr

{
    "wallets": {
        "defaultPassword": true,
        "mnemonicShown": false,
     },
    "backup": {
        "location": "...",
        "defaultLocation": true,
    }
}