Update message (what's new + link to release information)

[4rchim3d3s]

Feature description

Now when clicking on update we get this message:

I know this is a testing release but it would be great to always have infos inside this message about what new features got added. Something like the changelog here:

and a link (https://github.com/SkycoinProject/skywire-mainnet/releases/tag/v0.2.4-test-4bacd5e0) to more information.
Displaying only the link I think is easier than the whole changelog and may also be enough.

I guess this is not that much implemented yet, as there at the moment there should be a warning that this update should not be installed yet in this message. (Maybe will never be a problem in the future when the update mechanism is working.

Also the way the message is displayed is a lot of text. How about this format?:

Currently Installed: v0.2.3 (releasedate)
Available update:    v0.2.4 (releasedate) new features

Unstable update: v0.2.4-test (releasedate) (handle with care)

[jdknives]

Partially handled here github.com//pull/475

[jdknives]

Fixed with #506