Remove "bookmarks" from Project model

[maxceem]

We are moving links to be stored in the ProjectAttachmenets model instead of the bookmarks field in the Project model, see respective ticket appirio-tech/connect-app#3474.

As soon as we successfully transfer all the links in existent data to the new model and test it out for some time on DEV and PROD, we have to remove all the old logic regarding bookmarks.

[maxceem]

@vikasrohit do you think we can do this now? A new way of storing links in the attachments looks like working well. Or would you like to hold on it a little bit more?

[vikasrohit]

Yep, we can do it now. But lets schedule it for on release after.