Improvements for add bill form

[magshee]

As a user of PayForMe, I'd like two improvements in the "Add Bill" form, that will propably increase the quality of life.

1. The possibility to remember the last payer: Most of the time, I am using PayForMe to enter bills that I payed for. But since my name is always the second one on the selection above, for each new bill, I need to explicitly select my name again.
2. Autofocus the "What was paid?" field, to automatically open the keyboard.

I think there are more things that can be improved here, I just wanted to start with these two. Eventually other users have more ideas, that can be gathered in this ticket.

I am open for discussion on wether my two points are implemented, and how they should be implemented. Eventually I am also able to find some time to do them (hopefully quicker than my last PR ;))