Implement Bookmarks

[sm86]

At present, we have bookmarks in Nav Bar of Conference with no functionality.
Solution:

1. Add a button on Details of Event screen to bookmark it.
2. Save the data in the database SQLite.
3. Display results when Bookmarks is clicked.

Edge Case:
See that each bookmark is added only once to avoid repetition.
Enhancement:
Add a flag to know if the event is bookmarked already when you are looking at schedule itself.

Can I proceed with this work?