fix(icons): migrate conversation avatars to MDI icons

[Antreesy]

☑️ Resolves

• Move avatar icons to MDI appearance
• Fix contrast for the light theme

🖌️ UI Checklist

🖼️ Screenshots / Screencasts

🖼️ Making the light theme contrast

#999999 and #6B6B6B aligned with var(--color-text-maxcontrast)

🏚️ Before	🏡 After

🖼️ Switch to material design icons

🏚️ Before	🏡 After	🏚️ Before	🏡 After
Group	Group	Public	Public
Mail	Mail	Password	Password
Phone	Phone	File	File
User	User

🚧 Follow-up

• Replace public icon with group icon -> add "status-like" icon for indicating public conversations

🏁 Checklist

• 🌏 Tested with Chrome, Firefox and Safari or should not be risky to browser differences
• 🖌️ Design was reviewed, approved or inspired by the design team
• 📗 User documentation in https://github.com/nextcloud/documentation/tree/master/user_manual/talk has been updated or is not required

[szaimen]

New icons and colors look good to me now from design perspective :)
(didnt review the code and didnt test)

[jancborchardt]

@Antreesy good icon changes! One thing – after trying it out a bit more, it feels like the color scheme for the placeholders would look better the other way around between dark and light mode – with the new and better contrasting colors of course:

• Light mode: Light background and dark icon
• Dark mode: Dark background and light icon (as it was before)

Or was it switched around for specific reasons?

[Antreesy]

Or was it switched around for specific reasons?

Was aligned with colors used at server apps (Files):

[jancborchardt]

Ah right, and that was also for improved accessibility. All good then and thanks for the note @Antreesy! :)