Allow internal conversions using BlockTrades bridge, minor fixes to gateway page

[emfrias]

• Allow users to convert between BitShares asset types (e.g. BTS -> BITUSD) using the BlockTrades bridge on the Deposit/Withdraw page.
• Remember the last output coin type the user selected in the bridge for each input coin type, and provide a sensible default (BTC instead of AGRS) if they have never selected an output coin type before. This will help prevent a common user mistake.
• Fix a display bug that was clipping deposit addresses for the gateways