Bug: start price is not cleared out when swap tokens

# Description
When I fill in the start price field with a value (f.e. with a market price) and then swap tokens, the previous price value remains to be displayed in the start price field

# To Reproduce:
1. Go to Deploy tab
2. Pick a Token A and a Token B
3. Click on the market price value --> the value appears n the Start Price field
4. Swap the Token A and the Token B

# Actual Behavior
Value in the Start Price field remain to be the same

# Expected Behavior
Start price value is not cleared out when swap tokens

# Screenshots/Videos
https://drive.google.com/file/d/14iyl9gRV7pmDig3QefY38eysV1Au99EI/view

# Environment
**Desktop:**
- OS: Win 10
- Browsers: Chrome, Brave, FF
- Version 10/6/2020 11.00 AM UTC

Start in the Deploy tab and reproduce the flow by selecting Token A and Token B, applying the market price, then swapping the tokens. Trace the token-swap handling and verify that the Start Price field shows the intended cleared state after the swap.

Written by the indexing model from the issue text.