Consider using `.` prefix for hidden sphere files such as profile

[gordonbrander]

Currently we use underscore _ to denote "hidden file". However, using . would make us conformable to the way file systems hide files. That has some upsides.

This would require loosening the slashlink grammar to allow for . in slugs.

This would have the fringe benefit/tradeoff of allowing for file name extensions. These might be used as a hint for content-type in some scenarios.

[gordonbrander]

Another fringe benefit is that we don't have to escape . in SQL queries that filter out hidden files.