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
- Moves guest user caching from User class to app container for
simplicity.
- Updates test to use simpler $this->users->guest() method for
consistency.
- Streamlined helpers to avoid function overlap for simplicity.
- Extracted user profile dropdown while doing changes.
Dan Brown