Can't reuse names of deleted accounts

[malfynnction]

Describe the bug
If I want to reuse the name of a deleted account, I get the error message the account name must be unique for the budget

To Reproduce
Steps to reproduce the behavior:

1. Create a new account named "foo"
2. Delete that account
3. Try creating a new account named "foo"
4. See error

Expected behavior
Deleted accounts should not be considered for name uniqueness

[morremeyer]

This is trickier to solve than I thought at first. My two main points are:

• Soft-deleted resources need to be considered for uniqueness since they can be restored
• I do not think we should keep soft-deleted resources around forever. A trash bin mechanism where they get deleted after e.g. 30 days. The main reason why I implemented soft deletion in the beginning was to enable the "ooops, I deleted that accidentally, restore it now" workflow, not to keep them around forever.

With these, I arrive at the following:

• Resources continue to be soft-deleted
• After 30 days, they get cleaned up by a background task in the backend
• As described in Enable querying of deleted resources #252, it needs to be possible to get a list of deleted resources.
• As described in Enable restoring of deleted resources  #253, it needs to be possible to restore these
• Additionally, it should be possible to immediately delete a soft-deleted resource permanently via the API (needs a new issue or can be implemented via this issue)

@malfynnction What do you think about this? Should we discuss and document the frontend implementation for what would be needed here?

Thanks to @ekeih for discussing this with me from a user perspective.

[morremeyer]

This is both a bug and describes a possible enhancement:

Bug

There is currently no way via the API to re-use names since soft deleted resources cannot be restored or permanently deleted.

This will be fixed by allowing to query, un-delete and permanently delete soft-deleted resources.

Feature

Introduce the trashbin-like mechanism with an expiration of 30 days described above that automatically cleans up deleted resources.

This should be disabled by default for now, since enabling it will be a breaking change.

This is tracked in #1025 now.
#995 will only deal with fixing the bug