[etherpad] Move etherpad IDs to separate table (and implement public etherpad items)

To implement public pads, we should move the etherpad mappings to a dedicated table. This ensures that we can separate the returned data (instead of bundling it always in the extras).

Once the IDs are not part of the items, we can probably use the [pad public status flag](https://etherpad.org/doc/v1.8.18/#index_setpublicstatuspadid-publicstatus) to enable full access to the pads. Note that we still need to carefully separate uses cases that should only show the pad publicy in read-only mode (in which case we can use the [readOnlyID](https://etherpad.org/doc/v1.8.18/#index_getreadonlyidpadid) although we should never leak the actual pad ID), and cases where public open write access is allowed and the actual padID is used. This flag should be synced with the item public status.