Add installer downloader and in-app upgrades backend

[dlon]

This PR aims to introduce a minimal installer as well as most of the functionality for in-app upgrades (except UI). This is still a work in progress.

The main things that are missing:

• Obtain verified version info from stagemole the Mullvad API
• Implement UI correctly/fully
• Packaging/build scripts

The planned native module for the Electron frontend is not in scope for this PR.

Close DES-1685
Close DES-1684
Close DES-1641
Close DES-1640
Close DES-1571

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This change is 

[linear]

DES-1684 Create UI for macOS

DES-1685 Create UI for Windows

DES-1641 Make UI according to the design

DES-1640 Implement download and verification

DES-1571 Make downloader and verifier

[Serock3]

I get this warning when running cargo check

warning: profiles for the non root package will be ignored, specify profiles at the workspace root:
package:   C:\Users\Sebastian\Documents\Programming\mullvad\installer-downloader\Cargo.toml
workspace: C:\Users\Sebastian\Documents\Programming\mullvad\Cargo.toml

Is the opt level specified in installer-downloader being overwritten by the workspace level?

[Serock3]

I guess convert-assets.py is not supposed to be tracked in the repository, since we store the converted file, right?

[dlon]

I get this warning when running cargo check

warning: profiles for the non root package will be ignored, specify profiles at the workspace root:
package:   C:\Users\Sebastian\Documents\Programming\mullvad\installer-downloader\Cargo.toml
workspace: C:\Users\Sebastian\Documents\Programming\mullvad\Cargo.toml

Is the opt level specified in installer-downloader being overwritten by the workspace level?

It isn't being correctly applied. This is noted in the toml file. I think it'll be replaced by a build script.

[dlon]

I guess convert-assets.py is not supposed to be tracked in the repository, since we store the converted file, right?

I think it should because it makes it easier to regenerate them if the SVGs are edited.