fix: change the default image title (BACKPORT)

[DmytroAlipov]

This is a backport from the master branch. PR to master

The platform's default image title for staff is placeholder-faculty.png. However, in the default course, the image link still has title pl-faculty.png. Because of this, the default image is not displayed on the About course page. It is like this now:

For any created course where an image has not been set for the staff:

I manually tried to input the existing title of the image. Here's the result:

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[itsjeyd]

Thanks @DmytroAlipov! It looks like this is ready for review (but let me know if you were planning to work on it more).

@sarina Since you reviewed the main PR, would you mind having a quick look at this Palm backport as well?

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