add Saved Tweets tab to ProfileScreen

[johann-gambol]

[#537] Add Request: Saved tweets for profile

Add a 'Saved Tweets' tab to the ProfileScreen. To achieve this, I introduced a userId getter to the SavedTweet entity, which extracts the id from the content property. The ProfileSaved tab is more or less identical to the ProfileTweets and ProfileFollows views.

[jonjomckay]

Thanks for this too @johann-gambol! I made a change to extract the tweet user IDs into a database field, rather than doing it every time the page is loaded, for better efficiency.