move `SoftDeletedMetadataTable` to `common-lib`

[twrichards]

https://trello.com/c/DrGAH8Y0/893-turn-on-the-reaper

Following on from https://github.com/guardian/grid/pull/4143 (which moved 'reapable' eligibility logic into common) this PR moves the SoftDeletedMetadataTable to common (so that we can use it from both media-api and thrall in upcoming PR). It's in its own PR to make it easier to review.

Also removes usage of the global execution context, in favour of an implicit param for execution context.

IMPORTANT

This involves moving the dynamo.table.softDelete.metadata property from media-api.conf up to common.conf. Since this is a lazy val and only referenced in media-api, AFAICT nothing should break... but since it's going to be relied upon in https://github.com/guardian/grid/pull/4145 very soon and does no harm in common.conf it ought to be moved before this PR is merged.

Guardian checklist

• [x] TEST
• [x] PROD

[prout-bot]

Seen on usage, image-loader, leases, cropper, collections, media-api, kahuna (merged by @twrichards 13 minutes and 1 second ago) Please check your changes!

[prout-bot]

Seen on metadata-editor, thrall (merged by @twrichards 13 minutes and 8 seconds ago) Please check your changes!

[prout-bot]

Seen on auth (merged by @twrichards 13 minutes and 20 seconds ago) Please check your changes!