Allow creating `permalink` object tags on S3

[whatnick]

To manage the lifecycle of sharelink JSON object better in some deployments infrastructure engineers may choose to delete objects after a certain number of days. Users aiming to have long term persistence of certain sharelinks can create them with a permalink or similar S3 Object Tag and filter them out of the lifecycle rules.

Relevant change in the server code can be made here : https://github.com/TerriaJS/terriajs-server/blob/03c15ca8e9c137c12f8b8068108b468be6d661ae/lib/controllers/share.js#L112 and relevant UI options added to enable permalinks. Current behaviour is such that all links are permalinks and contents of the share url S3 bucket will grow infinitely over time. This may not be an issue except in very high traffic deployments.