Don't allow users to choose names that conflict with search syntax, like `any` or `none`, or names
that impersonate user levels, like `Admin`, `Moderator`, `Anonymous`, etc.
Features of AVIF include:
* Lossless and lossy compression.
* High dynamic range (HDR) images
* Wide color gamut images (i.e. 10- and 12-bit color depths)
* Transparency (through alpha planes).
* Animations (with an optional cover image).
* Auxiliary image sequences, where the file contains a single primary
image and a short secondary video, like Apple's Live Photos.
* Metadata rotation, mirroring, and cropping.
The AVIF format is still relatively new and some of these features aren't well
supported by browsers or other software:
* Animated AVIFs aren't supported by Firefox or by libvips.
* HDR images aren't supported by Firefox.
* Rotated, mirrored, and cropped AVIFs aren't supported by Firefox or Chrome.
* Image grids, where the file contains multiple images that are tiled
together into one big image, aren't supported by Firefox.
* AVIF as a whole has only been supported for a year or two by Chrome
and Firefox, and less than a year by Safari.
For these reasons, only basic AVIFs that don't use animation, rotation,
cropping, or image grids can be uploaded.
Add stricter username rules:
* Only allow usernames to contain basic letters, numbers, CJK characters, underscores, dashes and periods.
* Don't allow names to start or end with punctuation.
* Don't allow names to have multiple underscores in a row.
* Don't allow active users to have names that look like deleted users (e.g. "user_1234").
* Don't allow emoji or any other Unicode characters except for Chinese, Japanese, and Korean
characters. CJK characters are currently grandfathered in but will be disallowed in the future.
Users with an invalid name will be shown a permanent sitewide banner until they change their name.
The median username length is 8 characters. The 99% percentile is 18
characters. The 99.9% percentile is 24 characters. About 750 users have
a name more than 24 characters long.
This doesn't do anything about existing users with long usernames.
Note that this is the length in Unicode codepoints, not grapheme
clusters. Some Unicode characters and emoji may be a single glyph but
composed of multiple codepoints.
