Update to electron 7

[pronebird]

Describe what this PR changes. Why this is wanted. And, if needed, how it does it.

Git checklist:

• Describe the change in CHANGELOG.md under the [Unreleased] header.
• Check that commits follow the Mullvad coding guidelines

This PR updates Electron from 6.x to 7.1.10 which should fix a number of issues including crash during the app termination.

Due to API changes in Electron some of the calls had to be changed, highlight mode on tray icon on macOS is no longer handled by Electron so there is no way to control it anymore, so I had to remove the call to tray.setHightlightMode(), but that doesn't look bad on macOS so it's fine that we let the system to highlight the item on click, the highlight does not stay when the window is open.

Breaking changes document:
https://www.electronjs.org/docs/api/breaking-changes

Notable changes since v6:

• Fixed potential hang when sending syncronous IPC messages on process shutdown
• Fixed a crash that would occur when Notifications were closed in concert with app termination.
• Fixed incorrect size of windows on differently scaled monitors

See more: https://github.com/electron/electron/releases

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This change is 

[faern]

Looks good. Just make sure it's tested a little bit on all platforms before merged.

Reviewed 5 of 5 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved

[pronebird]

@faern could you please have a look on Linux?

Reviewable status: complete! all files reviewed, all discussions resolved