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
Previously we'd prevent caching of authed responses for security
(prevent back cache or proxy caching) but caching could still be an
issue in non-auth scenarios due to CSRF (eg. returning to login screen after
session expiry).
For #4600
