Setup auto update - Closes #196

[slaweet]

What was the problem?

Users stay stayed on older versions without knowing there is a new one.

How did I fix it?

By setting up electron-updater

How to test it?

• decrease version in app/package.json to 1.2.0
• build nano with npm run pack (On mac) or npm run pack:win (On Windows)
• install the built package from 'dist' folder
• open lisk nano
• wait for couple seconds
• there will be an alert like this:
• click "Update now"
• wait for a minute or two for the download (you can use that time to check version in about dialog or log in and check that "encrypt message" feature is present)
• there will be another alert when the download finishes, confirm it.
• the app restarts
• check version in about dialog or log in and check that "encrypt message" feature is no longer present

Review checklist

• The PR solves Notify users when there is new version available #196
• All new code is covered with unit tests
• All new code follows best practices
• Works on Mac and Windows (Linux will be handled separately Setup auto update on Linux #991)

[yasharAyari]

New version number doesn't exist in update popup message.

[ginacontrino]

@slaweet when I click on "update", it takes like 30 seconds or more before I get the notification "Updates downloaded, application will be quit for update...". This was quite confusing, because I was wondering if it's updating or not. Usually you get some kind of progress bar or some other kind of notification that something is actually happening.

Also, when I download the app, and I move it to the "Applications", but open it from here:

And then click "update", it is not actually updating. I, as a user, often open the app directly from that window, so I can imagine other people doing that as well.