Add option for marking bookmarks as unread by default

[ab623]

Fixes #669

This feature adds an option to the user profile to set the default state for Mark as unread.

• The migration does not update the current default, therefore there is no migration impact.
• I chose to make the field Mark as unread default, rather than a Mark as read default, this is so that it mirror the existing form. e.g if its checked in the setting it will be checked on the bookmarks form.

This is my first proper contribution to this project, so apologies if I have missed anything. I picked up this feature as it seemed like an easy one to implement.

[sissbruecker]

Thanks, looks good. Could use one more test.

[sissbruecker]

Thanks!