Remove empty Carrierwave directories when asset file is removed

[floehopper]

When we implemented #296, we didn't remove the directories as well as the files. This shouldn't cause any immediate problem, but it might be a bit confusing to people, so it would be good to stop the app leaving these directories lying around and to remove all the existing ones.

• Change Asset Manager app so that it removes empty CarrierWave directories when a new asset file is uploaded to S3 and removed from the filesystem.
• Remove empty CarrierWave directories for existing assets which have already been uploaded to S3 and deleted from the filesystem.

[floehopper]

Note that for completeness, we may want to remove the empty directories from the govuk-attachments-production S3 bucket which prior to alphagov/govuk-puppet#7019 was sync-ing Asset Manager assets every night.

[floehopper]

I've opened #390 to address this issue for new assets in order that the disk usage for Asset Manager assets doesn't increase and trigger the asset master/slave disk usage alert once alphagov/govuk-puppet#7016 has been applied and Asset Manager assets are no longer being sync-ed from asset master to asset slaves.

We will need to decide what to do about existing empty directories separately.

[floehopper]

I've just merged #390.

[floehopper]

#390 has just been deployed to staging (@ ~10:35).

[floehopper]

#390 has just been deployed to production (@ ~10:45).

[floehopper]

We still need to remove the existing empty directories. I'm going to add a checklist to the description to make that more obvious.

[floehopper]

I'm going to move this back in to the To Do column so we can decide when/if to prioritise the remaining work.

[floehopper]

Since we've reverted #390 in #410, I'm going to uncheck the first checkbox in the description of this issue.

[floehopper]

I'm going to un-assign myself from this issue since I'm no longer actively working on it.