Within the responsibility of the theme service instead
of being part of the app configuration.

Adds a registration system via the logical theme system, to tell
BookStack about views to render before or after a specific template
is included in the system.

Updated function name also.

Changed the system out to be a theme event instead of method, to align
with other registration events, and so that the theme view work can
better be contained in its own class.

Not yet tested at all, either manually or via PHPUnit

- Added (limited) redirect handling to module downloads.
- Adjusted wording/text for consistency and clarity.
- Fixed scenarios where process was not stopped on error.
- Fixed module folder creation check/logic.
- Added better failed request handling to module downloads.
- Updated download response streaming to monitor/limit download size.

- Updated the system to work with modules.
- Updated module docs to consider namespacing.
- Fixed view loading and registration event ordering.
- Fixed checking if views are registered.

Split out each sidebar block into their own template for easier
customization of those elements, and less code to manage when overriding
the parent show/index views.

…development

Further theme system developments