fix(Conversation): adjust design of icons

[ShGKme]

☑️ Resolves

• Fix Conversation list item pop-up menu icon design #12781

🖌️ UI Checklist

🖼️ Screenshots / Screencasts

🏚️ Before	🏡 After

🚧 Tasks

• rename imported icons to Icon*
• make "Mark as read" icons filled
• use content-copy icon for copy
• make all icons in menu 16px

🏁 Checklist

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

[AndyScherzinger]

Woohoo! Thanks a lot @ShGKme for the harmonizing of the icons 🙏

[ShGKme]

Follow up: also change icons in the message bar