add bookmark functionality #330

[only-dev-time]

Adds the function to save bookmarks for posts and comments.

Changes in detail:

• Added a new table hive_bookmarks
• Increase of the DB version to 21
• Processing of custom_json operations (structure #330) for adding and removing a bookmark
• Added a new request bridge.get_bookmarked_posts
• Post data returns an additional field bookmarked_by containing the accounts that have bookmarked the post or comment.

No resync is required.

[only-dev-time]

I rebased the feature branch and removed some commits that are not required for the final version. This should make the review a little easier.