Adds background image and Applications shortcut to macOS disk image

[holmesworcester]

Many users have reported confusion with the Cannot create directory: /Volumes/Ricochet/config.ricochet error they see when neglecting to drag Ricochet.app out of the disk image. See #112, #561, #510, #491, etc.

Ricochet is still an amazing proof of what's possible for private, peer-to-peer chat and it's a shame that so many Mac users will hit this bug, so I got inspired and fixed it. I tested Ricochet with two friends and both experienced this issue and assumed Ricochet simply didn't work. (One friend is a junior software engineer, and the other is very technical. It seems like that this bug is preventing a lot of users from trying Ricochet!)

This PR modifies release_osx.sh to add a shortcut to the user's Applications folder and a background image reminding the user to drag the file, using the create-dmg script and following the default UX of electron builder. It uses a weird-and-tricky-to-create background image (this one) from the electron builder project. Electron-builder is available under an MIT license, so the use of this image should not present any problems. Mac users will see the following when they download Ricochet:

My own peer-to-peer messaging project Zbay uses the same default electron-builder DMG layout as the one above, and user typically drag the app to their Applications folder. It would be possible to add branding here, but that seems like overkill.

I'm a junior developer, so apologies for the funky commit history, but it works and seems sound. I wasn't sure if this commit made sense, but it was necessary for me to get the release_osx.sh working locally so I figured it might be helpful to others.

[holmesworcester]

Also, it may be worth testing distributing Ricochet as a .zip file. I think some browsers unzip it by default to the downloads folder, at which point it would be impossible for users to make the mistake this PR addresses. Many heavily user-tested apps (Spotify, I think?) use this approach, but it's a bit less common. This might be worth trying too and I'd be up for trying it if the PR would be accepted.

[cypherbits]

Hello @holmesworcester , Ricochet IM is an abandoned project. This repository is not updated.

There was an attempt to renew it on https://github.com/blueprint-freespeech/ricochet-refresh https://ricochetrefresh.net/ but it is already abandonware too.

We look forward to Cwitch.im, based on Ricochet but written in Golang.

[holmesworcester]

@cypherbits bummer! let me know if the above packaging work would be helpful to Cwtch!

[m-simonelli]

Hi @holmesworcester, We at blueprint are actively working on ricochet refresh, so feel free to make a pull request to our build repo over at https://github.com/blueprint-freespeech/ricochet-build