Frontend supports only 18 decimal tokens

[mariano-aguero]

Closes #516

Includes:

• Pass base token's decimals to buy_sell.tsx's BigNumberInput, so the amount obtained is correctly expressed in base units.
• Fix makerAssetAmount calculation in the function buildLimitOrder from src/util/orders.ts (should be expressed in the taker asset's base units).
• Fix calculation of price in buildLimitOrder from src/util/ui_orders.ts.
• Adapt UI components that didn't considered decimals other than 18 to show a price (e.g. the markets dropddown).
• Fix failing tests due to changes in src/util/orders.ts.

[unjapones]

Hey Mariano, I tested this yesterday and found out that there might be a problem withe the units handling. I created a Sell limit order with the following data:

• base token: DGD, 9 decimals
• quote token: WETH, 18 decimals
• value in "Amount" input: 2
• value in "Price per token input: 1.5

The above generated the following when MetaMask asked me to sign the order (screenshot):

I believe there is a problem around the amounts and that the expected SignedOrder should end up with the following values:

makerAssetAmount: 2000000000,
takerAssetAmount: 3000000000000000000

makerAssetAmount should be expressed in base units of the maker asset, and the same goes for takerAssetAmount (which has more zeros because WETH has more decimals).

Please confirm that you see the same thing. We'll continue improving this PR's branch.

[mariano-aguero]

LGTM @unjapones , I think is ready to merge!