Add "Check for Updates" menu item

[mirka]

Closes #1041

Currently, the app checks for updates on launch, downloads it in the background, and only prompts the user when the download is completed. This was a good "minimum viable" UX because the built-in Electron dialogs don't allow dynamic updating to show download progress, and the app takes a significant amount of time to download.

This PR enhances that minimum implementation by adding a "Check for Updates" menu item. Users can now manually check if they're using the latest version, and this also lays the groundwork for a "don't check for updates" option (#842) that we should add next.

Test builds

Should say that no updates are available:

• Windows
• Linux
• Mac

Note: The icon in the Linux dialog is broken. This is a known bug, and will be fixed when we update to Electron >=2.0.13.

Should download an update:

• Windows
• Linux
• Mac (For it to actually install the downloaded update, the Simplenote app must be in the Applications folder. FYI the downloaded update is saved in ~/Library/Application Support/Simplenote/__update__/, if you would like to remove it.)

[roundhill]

For a bit on Windows 7, I got this message:

After maybe 10 seconds, it did download the update though 🤔 Let me know if you can reproduce that.

[mirka]

After maybe 10 seconds, it did download the update though 🤔 Let me know if you can reproduce that.

This may happen if the GitHub API call flaked out on the user-initiated check, but succeeded on the check that happens automatically on application launch. In this case, you would not have seen a download progress window — just a dialog that says an update is ready to be installed.

[roundhill]

This may happen if the GitHub API call flaked out on the user-initiated check, but succeeded on the check that happens automatically on application launch. In this case, you would not have seen a download progress window — just a dialog that says an update is ready to be installed.

Ah ok 👍