[feature] Allow management of UpgradeOperation objects in the admin

[okraits]

Right now, there's no possibility to modify or delete UpgradeOperation objects in the admin interface:

https://github.com/openwisp/openwisp-firmware-upgrader/blob/master/openwisp_firmware_upgrader/admin.py#L168

I had an in-progress upgrade operation but it didn't continue because of an exception. It didn't fail either. So it was stuck.

So we should provide the possibility to:

• cancel an in-progress upgrade operation or set its status to failed
• delete single upgrade operations
• clear the list of upgrade operations

[pniaps]

Same needed here.
It is very annoying to test and not be able to erase test traces.