Allow configuring a custom MQTT server

[swentel]

Follow up to #214

For custom servers:

• allow adding a custom uri, and optionally username and password
• subscribe to something - need to figure out what exactly this is

See https://github.com/owntracks/android for code

[dshanske]

A topic, perhaps indigenous ?

[swentel]

Yep makes sense. To configure I currently see following input fields:

• uri
• login (optional)
• password (optional)

That should do it right ? (apart then for listeners of course)

[dshanske]

Port. SSL uses a different port than non ssl

[dshanske]

That might be in the Uri

[swentel]

List of brokers I could test with: https://mntolia.com/10-free-public-private-mqtt-brokers-for-testing-prototyping/

[swentel]

Gave this some more thoughts and I'm not going to support this for now.

Pull requests are welcome, but this one is so rare that it's a bit overkill to support this.

[swentel]

Reopening, there's time to implement this!