add bookmarks functionality

[only-dev-time]

I would like to suggest a new feature for Hivemind and steemit.com: Bookmarks.

For each post and comment, users can set a bookmark. The functionality is provided by a custom_json operation on the blockchain. This means that other frontends can also process and display the bookmarks.

The operation is structured as follows and is to be processed by hivemind:

Setting the bookmark

{
  "required_auths": [],
  "required_posting_auths": [
    "moecki"
  ],
  "id": "follow",
  "json": "[
    'bookmark', {
      'account': 'moecki',
      'author': 'moecki',
      'permlink': 'moecki-witness-update-february-2024',
      'action': 'add',
      'category':''
    }
  ]"
}

Removing the bookmark

{
  "required_auths": [],
  "required_posting_auths": [
    "moecki"
  ],
  "id": "follow",
  "json": "[
    'bookmark', {
      'account': 'moecki',
      'author': 'moecki',
      'permlink': 'moecki-witness-update-february-2024',
      'action': 'remove',
      'category':''
    }
  ]"
}

A new table hive_bookmarks is required to save the bookmarks. A database migration is therefore necessary.

First thoughts: https://steemit.com/hive-151113/@moecki/what-do-you-think-about-bookmarks-in-steemit
Demo: https://steemit.com/hive-146118/@moecki/bookmarks-fuer-steemit-com-exklusive-einblicke-vor-dem-finale