Fix username and avatar for external users

[nickvergessen]

Fixnextcloud/activity#155

Steps

1. Share a folder as public link
2. As a visitor of the link create a folder and upload a file
3. As visitor of the link move the file into the folder
4. As link owner open your activity stream

Expected:

"remote user" moved aviatorjk_2112.jpg to test

Actual

[nickvergessen]

Would backport for 12.0.1 only, not important enough...

[MorrisJobke]

Tested and works 👍