Markup /oc/19

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New features:
 - **Anonymous threads**: posts in an anonymous thread do not have the author's account linked; they are also not visible on user pages.
 - **Restricted threads**: visible only to logged-in and activated users.
 - **Inline references to uploaded attachments**: attachments can be referenced using `![](<index>)`, where `<index>` designates the specific attachment to use.
 - **Button to show/hide mod controls**: located in the top bar, this button toggles the visibility of control buttons, IP hashes, etc.

Misc. improvements:
 - Inline images now have the `no-referrer` attribute; the Referer header will not be sent when downloading an image from an external source.
 - Removed the flickering effect caused by post timestamps being replaced with relative times during page refreshes.