Implement shared ID for public links and shares

[jessegeens]

Through this PR, a new table share_ids is added to the database, which contains the sequence of IDs that are shared between public links and shares:

+-------+-----------------+------+-----+---------+----------------+
| Field | Type            | Null | Key | Default | Extra          |
+-------+-----------------+------+-----+---------+----------------+
| id    | bigint unsigned | NO   | PRI | NULL    | auto_increment |
+-------+-----------------+------+-----+---------+----------------+

This is required to comply with Web's requirement of having unique IDs across the different share types.

The shares table (and similaryly the public_links table) remains unchanged, except for the facts that:

• Id is now a Foreign Key to share_ids.id
• Id is no longer the primary key. Instead the primary key is a composite key on (storage_id, inode, share_with)

+----------------------+------------------+------+-----+---------+-------+
| Field                | Type             | Null | Key | Default | Extra |
+----------------------+------------------+------+-----+---------+-------+
| id                   | bigint unsigned  | YES  | UNI | NULL    |       |
| created_at           | datetime(3)      | YES  |     | NULL    |       |
| updated_at           | datetime(3)      | YES  |     | NULL    |       |
| deleted_at           | datetime(3)      | YES  | MUL | NULL    |       |
| uid_owner            | varchar(64)      | YES  |     | NULL    |       |
| uid_initiator        | varchar(64)      | YES  |     | NULL    |       |
| item_type            | varchar(16)      | YES  |     | NULL    |       |
| initial_path         | longtext         | YES  |     | NULL    |       |
| inode                | varchar(32)      | NO   | PRI | NULL    |       |
| instance             | varchar(32)      | NO   | PRI | NULL    |       |
| permissions          | tinyint unsigned | YES  |     | NULL    |       |
| orphan               | tinyint(1)       | YES  |     | NULL    |       |
| expiration           | datetime(3)      | YES  |     | NULL    |       |
| share_with           | varchar(255)     | NO   | PRI | NULL    |       |
| shared_with_is_group | tinyint(1)       | YES  |     | NULL    |       |
| description          | varchar(1024)    | YES  |     | NULL    |       |
+----------------------+------------------+------+-----+---------+-------+

| shares | CREATE TABLE `shares` (
  `id` bigint unsigned DEFAULT NULL,
  `created_at` datetime(3) DEFAULT NULL,
...
  `shared_with_is_group` tinyint(1) DEFAULT NULL,
  `description` varchar(1024) DEFAULT NULL,
  PRIMARY KEY (`inode`,`instance`,`share_with`),
  UNIQUE KEY `idx_shares_id` (`id`),
  KEY `idx_shares_deleted_at` (`deleted_at`),
  KEY `i_share_with` (`share_with`),
  CONSTRAINT `fk_shares_share_id` FOREIGN KEY (`id`) REFERENCES `share_ids` (`id`) ON DELETE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci |

[glpatcern]

All good for me, there's just a trivial change