techdebt(LoadingPlaceholder) - rewrite skeleton component

[Antreesy]

☑️ Resolves

• Replaces pulse animation with wave animation (see article about effect on user experience)
• Enable hardware acceleration (was broken before)
• Adjust styles to the existing components

🖼️ Screenshots

🏚️ Before

Participants had different component as placeholder:

skeleton-pulse.webm

🏡 After

Animation flow:

skeleton-wave.webm

In normal colors:

skeleton-light-theme.webm

skeleton-dark-theme.webm

🚧 Tasks

• Manual testing
• Code review
• Design review

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not required
• 📘 API documentation in docs/ has been updated or is not required
• 📗 User documentation in https://github.com/nextcloud/documentation/tree/master/user_manual/talk has been updated or is not required
• 🔖 Capability is added or not needed

[szaimen]

Looks very nice from design perspective! 🚀

Does it also look good in dark mode? (was a problem until recently IIRC)...

[Antreesy]

Does it also look good in dark mode? (was a problem until recently IIRC)...

Have updated screencasts for both dark and light themes, please see in the root description

UPD: rebased onto master, increased wave width and speed a bit

[DorraJaouad]

Overall, it's better than before 🦅. The animation looks fine and code-wise, it is more dynamic and concise. Just a tweak suggestion.

[Antreesy]

Rebased onto master with post-review fix, no other changes

[Antreesy]

/backport to stable27