Replace the Silk icons in Attachment Selector with the Font Awesome

Steps to reproduce

1. Click on avatar of the logged-in user (or go to the user's profile page)
2. Click on "Change Photo" button, under the avatar image, above the "Settings" panel
3. Observe the icons on the Attachment Selector modal

Expected results

All icons are part of the Font Awesome set (which is currently enforced on the wiki level).

Actual results

Some icons are part of the Silk Icons theme set: the green '+' one before the "Add" in the upper left, the "Select" (green check mark) and "Delete" (red "x") on the uploaded images (please see the attached screenshot).