nextcloud: non-interactive post-install completion

[julianfoad]

This proposal reduces the Nextcloud post-install requirements to a single, non-interactive step.

It replaces the previous print-nextcloud-db-credentials and interactive GUI setup step and adjust-nextcloud-config with a single non-interactive step, invoked with tag post-install-nextcloud (or post-install-all).

In doing so, it also resolves the following issues:

• Nextcloud not working with matrix-docker-ansible-deploy and non-root nextcloud_path_prefix #66, installing mash-nextcloud as subdirectory, not subdomain, breaks urls #121: problems with interactive installer assets at non-root base path (by removing it)
• "just run-tags adjust-nextcloud-config" should probably also update trusted_domains #131: updating the trusted_domains config (by automating it)

It incidentally introduces the idea of a common post-install-all tag. This could be omitted at this time.

Patches (branches):

• my ansible-role-nextcloud branch (to pull it: cd .../ansible-role-nextcloud && git pull https://lab.trax.im/fork/github/mother-of-all-self-hosting/ansible-role-nextcloud.git julianfoad-nextcloud-post-install)
• update to docs/services/nextcloud.md (to pull it: cd .../mash-playbook && git pull https://lab.trax.im/fork/github/mother-of-all-self-hosting/mash-playbook.git julianfoad-nextcloud-post-install)