Update icons in buttons and menus

[yexingsha]

• The new pdf_raw_reader icon in reader top bar looks out of place. Let's replace it with a 24px icon in material symbol style:
  

Also, with the addition of menu options to download or remove attachments for an entire collection, let's make these icons consistent with the icons used for individual items. Use material symbols, Fill = Off, Weight = 300, Grade = 0, Optical Size = 24px, Style = Material Symbols (new) + Outlined:

• Use download for both downloading individual attachments and downloading attachments for an entire collection.
• Use file_download_off for both removing individual attachment downloads and removing downloads for an entire collection.