chore: migrate to AppImage and publish with electron-builder on Travis

[IGassmann]

This closes #332 and closes #966.

[kauffj]

@JoshStrobl apologies if this is a redundant question (I have this feeling I've asked this), but for your purposes does this switch to AppImage: a) help, b) hurt, or c) doesn't matter?

[JoshStrobl]

@kauffj It doesn't affect packaging in my case. ("Doesn't matter.")

[tzarebczan]

@IGassmann per our convo, let's make sure the daemon is signed: lbryio/lbry-sdk#811

[probonopd]

Now that this has been merged, where did the AppImage go?

[kauffj]

@seanyesmunt do you know what happened to AppImage builds or what it would take to re-enable these?

[tzarebczan]

We only temporarily tested with these and decided not to go with them at the time because it required setting the executable properties manually before installing.

With syncing enabled now, I think it will be a good time to check out appimage and snaps.

[probonopd]

If you would like to test, here is an example AppImage I've made to be able tor run LBRY until an official AppImage becomes available:

https://github.com/probonopd/LBRY.AppImage/releases/tag/continuous

Before you can run an AppImage, you need to make it executable. This is a Linux security feature. There are three main ways to make an AppImage executable:

1. With the GUI

2. On the command line

chmod a+x Some.AppImage

3. Automatically with the optional appimaged daemon

If you would like to have all AppImages be executable automatically, you can install the optional appimaged daemon. It will automatically add downloaded AppImages to the menu and make them executable for you. It can be downloaded from https://github.com/AppImage/AppImageKit/releases or installed from your distribution.

You can see the .travis.yml I used to make the AppImage here:

https://github.com/probonopd/LBRY.AppImage/blob/master/.travis.yml

If you haven't seen it, also check out this: