Move user_external and external to separate repos

[PVince81]

The apps user_external and external should be released on the marketplace.

The first step is to move these to separate repositories.

[PVince81]

• external to separate repo: https://github.com/owncloud/external
• user_external to separate repo

[PVince81]

second one to be done when we have a reason to touch it

[PVince81]

I see we still bundle user_external and will need to touch it as part of Semver to increase its max-version.

Before doing so I suggest we move it to a separate repository in which we can then follow the process with release tickets, etc.

This way we can then archive the "apps" repository.

[PVince81]

@DeepDiver1975 ^

[PVince81]

user_external: https://github.com/owncloud/user_external

next steps:

• delete app from repo on master: Remove user_external app #2232
• delete app from repo on stable10: [stable10] Remove user_external app #2233
• archive "apps" repo.

or archive directly ? @DeepDiver1975

[PVince81]

any services that need to be migrated to the new repos ?

[PVince81]

• central post about archival: https://central.owncloud.org/t/archiving-the-owncloud-apps-repository/16762

[PVince81]

• archival notice in README: Update README for archival #2234