Add occ files:remove-storage command

[mmattel]

Referencing: owncloud/core#40779 (Add command to remove obsolete storages from the filecache)

Adding the occ command referenced by the core PR.

Merge only when the referenced core PR got merged.

NO backport, master only

@jvillafanez pls check if correct, all descriptions are here in the admin docs as it is no config command.

[jvillafanez]

We might need to add some usage examples to clarify the expectations.

Assuming you don't know what storages you could remove, you should use occ files:remove-storage --show-candidates first to get a list of possible obsolete storages. From there, you can use occ files:remove-storage <numeric_id> to remove that storage.

[mmattel]

We might need to add some usage examples to clarify the expectations

Will do.

[mmattel]

I added an example, but it is likely that the table header names will change. Waiting for the core PR.

[mmattel]

@jvillafanez I have updated the example table according the changes you made in the core PR. Note that from my side the PR is ready for final review.

[jvillafanez]

Content-wise looks good. Just a couple of minor things.

[phil-davis]

Text looks OK, assuming that this is the command interface that is finally merged in core.