Don't backup _data temporary folder (closes #17)

[JimboJoe]

Problem

_data folder is backed-up, whereas it's a temporary folder, duplicating the upload folder which contains every needed data (see #17).

Solution

Don't backup the _data folder. The only drawback is that Piwigo will need to regenerate temporary data (thumbnails, different photos sizes) on the fly (as it already does when the photo is viewed/browsed for the first time).

PR Status

Work finished. Package_check, basic tests and upgrade from last version OK.
Could be reviewed and tested.

Validation

• [x] Upgrade previous version :
• [x] Code review : Maniack C
• [x] Approval (LGTM) : Maniack C
• [x] Approval (LGTM) :
• CI succeeded :
  When the PR is marked as ready to merge, you have to wait for 3 days before really merge it.

[maniackcrudelis]

LGTM and code review OK.

[JimboJoe]

maybe @LeOSW42 could try the upgrade...?

[LeOSW42]

Upgrade an backup worked fine. Restore not tested, but seems ok.

[JimboJoe]

Thanks! So, let's merge in 3 days 👍