[diskette_dump] Save dump created from commandline as a DumpFile object

[sveetch]

Is your feature request related to a problem? Please describe.

Currently, creating a dump from commandline diskette_dump does not create any DumpFile entry, it justs create the dump that won't be retrieved from the admin and so:

• Dump file can not be retrieved from admin and so can't be purged;
• Dump file can not be downloaded from admin;

Describe the solution you'd like

We should add an argument --save to the commandline. When used it will create a proper DumpFile for the created dump archive. This should be done once after the dump has been well created, the DumpFile will be directly made with status "processed".

It should also perform the deprecation and purge processes but obviously not the admin dump process.