feat: add importer app

[JammingBen]

refs owncloud/web#11698

[CLAassistant]

All committers have signed the CLA.

[pascalwengerter]

@AlexAndBear we have some dev docs about this which is ok for devs, but we must have a description for admins in the admin docs. that is a different audience...

I generally agree but don't understand why this is a discussion that's happening in a pull request and not an issue opened in the relevant (docs) repository?

[kulmann]

It's not documented in the admin docs because it's a very quickly moving topic and evolving fast. Documenting it twice (dev and admin) would just be what I call ABM (job-creation scheme). Having a simple link to the dev docs section and 1-2 sentences why it's not in the admin docs would be a good idea for the admin docs though.

[mmattel]

We have to discuss this in more detail. I know things are moving fast, but I am looking forward to the coming stable release. Currently all stuff on dev docs is master and does therefore not hard align with versioned stuff like when we do a release. I am addressing this issue in particular.

[AlexAndBear]

Not in this PR. I marked most of the conversation as off topic. @mmattel please open an issue in the docs repo and mark the relevant people you need input from.

[kulmann]

Tiny idea for improvement about the dev stack integration... thanks for moving all that over from the web repo 😍