Apply new loading animation

[yuya-oc]

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Summary
Apply new loading animation.

Issue link
#748

Test Cases

1. Open the desktop app.
2. The animation should appear while loading the webapp.

Additional Notes
https://circleci.com/gh/yuya-oc/desktop/804#artifacts

[amyblais]

Works on Windows, I'll check with Mike if he would be able to apply a continuous animation on the icon soon or if this will be part of other improvements on the icon for later.

[amyblais]

@yuya-oc Mike just updated the animation, please see the ticket: https://mattermost.atlassian.net/browse/MM-8484

[yuya-oc]

@amyblais Thanks! Updated: https://circleci.com/gh/yuya-oc/desktop/809#artifacts

I could get fine result on my monitor, 150% of DPI scaling setting.

150%:

150% (without 2x image):

[amyblais]

Thank you @yuya-oc! I tested on Windows and it looks good - I think we are ready to merge this PR and get ready to cut RC-1.