Settings: Overhaul the Security page

This improves the organization of this page, which is starting to become a bit of a mess. The "Hide images and video events" option is moved here, and the page is rebranded accordingly for "Security & Safety". Unnecessary headings are removed, and the ignored users button is moved to the top of the page.

Explanations for the import/keys buttons are added. The key display is removed as it's not useful for the user (because they don't know what to do with it) nor developers (because you can't copy it.) We can add it back somewhere else.

Before	After

This has the added benefit of making the whole page fit in the default settings window size too.