Change "Remove" button to "Unfeatured" in featured articles admin view - see PR #5485

# Summary
With this PR, the button "Remove" in the featured articles view of the article manager is changed to a button "Unfeatured" as added with PR #5485 for the articles list and as discussed there.

The function of the button stays the same, but text and icon will be less misleading.
# Dependencies
For the Hathor template, the changes in CSS from PR #5485 are required to see the correct icon for the button.

If the ongoing discussion (see PR #5485) will result in changing the UNFEATURED text from "Unfeatured" to e.g. "Unfeature" or something else, this will be handled with a separate PR.
# Testing
Check label, icon and function of the button right beside the "Unpublish" button on the featured articles view of the article manager.
## Before applying patch with this PR
Label = "Remove", icon is an "X", with thie button selected featured articles will be changed so they are not featured anymore and so disappear from this list (but still are there, check in article manager's articles view)
## After applying patch
Label = "Unfeatured", icon is the same as used in the articles list for unfeatured status of articles (unfilled star), functions is unchanged