Add feature: display friend icons in minimized display of transactions

[Felicious]

Referencing #31, it would be nice that, once friend icons are added, their icons are displayed in the minimized view of a Transaction to preview who participates in this transaction.

Will need to determine:

1. A function to generate a list of unique friends from both payers and ppl the bill is split betw (to display these)
   

2. Possibly consider using an additional, empty placeholder at the end so that the icons aren't smushed next to the cost like in the "Fugetsu" example above