Markup /oc/166

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Gotta revive the thread since a lot have changed in the meantime, now that the zarty also uses it I had to add many features to facilitate the migration, but also a few others to close the feature gap with Vichan.

New features (since the [last post](https://ocgemerald.club/oc/4#post-oc-19)):
 - **stats panel on the home page**: showing the number of posts, users, total file size etc.
 - **LaTeX rendering, extended Markdown syntax**: checkboxes, heading references, spoilers, blue text
 - **login redirects**: opening a link that requires being logged-in redirects to the login page and then back to the original destination
 - **configurable default forum view**: either the thread list, index, or catalog, required for Vichan compatibility
 - **user mentions**: using the `@user` syntax
 - **ability to paste/drop files on the post form**: with inline references added automatically when dropped on the text area
 - **automatic captcha on abnormal CPU usage**: enabled by monitoring CPU usage in the background, should detect spam automatically
 - **viewing the original post markup**: the link is present in the post menu
 - **Vichan imports**: importing posts, IP notes, pages, and bans
 - **upload progress when making a new post**: visible on the submit button
 - **marking replies to OP**: `(OP)` visible after the reply
 - **random 404 images**: a new images can be added in the `404` directory
 - **latest posts list on the home page**: I should make this one configurable though, doesn't look good here
 - **extended allowed files list**: FLAC, OGG, ZIP, TAR, XCF, SWF, M4V, BMP
 - **simple bot detection**: most of the bots self-identify as such, some can be detected by inconsistent headers, but there are still many that aren't identified for now; this also affects thread view updating (configurable whether include bots or not), the online user list 
 - **search improvements**: including highlighting the searched text within the found posts
 - **thread ordering**: available in catalog and thread list view
 - **user flags**: also known as rizz/vril
 - **oekaki**: self-explanatory
 - **multiple appeals per ban**: with a 1 week timeout by default, configurable
 - **forum, thread, attachment hiding**: there are buttons to filter out threads from a board in overboard, hide threads, gray out images
 - **configurable bookmark and edit button positions**: available in the settings tab

Misc. improvements:
 - missing icons for restricted, anonymous threads and threads with proxy status visible added
 - various CSS improvements, including extracting the common CSS part to a separate file, delegating colors etc. to theme files
 - current version is now shown in the footer
 - captcha is possible to pass by clicking anywhere when the captcha images are missing
 - limit the max age of deleted posts when mass deleting posts by IP or user
 - migrated the HTML templates to a different library for 2-5x rendering speed-up
 - all external processes used for thumbnails, exif removal etc. are now executed within a sandbox
 - changed the URL schema, `/forum/`, `/thread/` fragments were rather useless
 - placeholders for deleted files instead of nothing
 - a ton of performance improvements
 - new diagnostic administration links for monitoring resource usage, file consistency and removing stray files (which I needed to remove when working on Vichan import) etc.
 - fixes for bugs with embeds, attachments