Removed default public visibility for images at the driver level,
leaving only doing this as a specific action in the logic.
Added try/catch around permission setting so that
permission-incompatible environments won't fatally fail, but instead
log a warning.
Tested via a google cloud storage bucket FUSE mount, mounted under another
user but with open 777 permissions.
Related to #5269
Allows files to be placed within a "public" folder within a theme
directory which the contents of will served by BookStack for access.
- Only "web safe" content-types are provided.
- A static 1 day cache time it set on served files.
For #3904
