Mailbox: Add a collect-all button

[jakmeier]

Description

Add a button to the rewards view that collects all available rewards at once, instead of going though them one-by-one.

Implementation

The API structure already allows to collect multiple reports at once. To avoid changes in the backend, we should just reuse the same API call with an explicit list of report ids.

The button should be somewhere on the Mailbox view and maybe also linked to a hotkey binding but this is completely up to the implementer.