delete category / feed: UNDO

[erwin]

Provide a mechanism to somehow recover a deleted category / feed.

Mutiple solutions are possible:

- script to retrieve from latest DB backup. (advantage: no extra features required when deleting; disadvantage: all changes since latest backup are lost)

- move category / feeds to a sort of 'trash' (e.g. in sql format, csv, database table, ...)

- don't actually delete categories but set a removed state. (cleanup can happen after a certain period of time)

[erwin]

This is a test of the trac notifications.

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[erwin]

It is probably best to use 'removed state' in categories and feed.

Since this will have a major impact on the code, move this issue to M8