Combined script cleanup and upgrade

[PrivatePuffin]

This PR combines some of the work from my previous PR's and builds on it to improve the script further and show why certain choices where made.

• It renames "configs" to "includes (from #93)
• Adds clear chapter defining comments to define different code structures (from #94)
• It merges similair code where possible (from #94)
• It moves all nextcloud-related mountpoint defaults from ${POOL_PATH}/ to ${POOL_PATH}/nextcloud/
• It adds a mountpoint from ${POOL_PATH}/nextcloud/config to /usr/local/www/nextcloud/config
• It adds config option, var and tests for the new mountpoint
• It edits the README accordingly

These options combined, open up some options for persistent reinstall (reinstalling while keeping config, DB and files). This is not yet in this PR, but thats relatively doable to add.

[PrivatePuffin]

Somewhat of a beta that enables reinstallation while keeping config and files, is available here: https://github.com/Ornias1993/freenas-iocage-nextcloud/tree/enable_reinstall

Another WIP (more like alpha) of mine to support external database servers: https://github.com/Ornias1993/freenas-iocage-nextcloud/tree/external_databases

(both based on the work done in this PR)