[FIX] Fix currency choice restriction with federation.

Previously the account_currencies restriction would overwrite any federation currency restriction. This patch implements the correct behavior where the Ripple network currency restrictions are intersected with the federation restrictions.

Also adds a loading message while currency restrictions are being determined.

Fixes a race condition where an old account_currencies request can succeed later, even if the destination has since changed.
[TEST] Update test cases for new currency choices logic.
Merge branch 'hotfix/1.0.5-1'