Cake Day Reminder: add option to be notified of other users' cake days

[Eiion]

Is your enhancement request related to a problem? Please describe.
No - it's simply a nice to have feature that reminds us of friend's cake days (after specifying their usernames in the settings).

Describe the solution you'd like.
A setting is added to 1.4 Cake Day Reminder - 1.4.3 Friend's Cake Days Reminder. In its settings there's a field where we input names of users we want to be reminded of their cake days.

Additionally a button could be added on user profile pages once 1.4.3 is activated that allows us to add users to the list with a simple click.