[Gnosis]: Show a better price estimate on transactions

[evgenyboxer]

Description

Following Gnosis team feedback, it was noted that the amount estimate shown upon trade confirmation does not include slippage/fees - this PR solves this.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other

How should this be tested?

Please provide instructions so we can test. Please also list any relevant details for your test configuration.

• Make an ERC20 <> ERC20 trade and check the estimated amount

Checklist:

• I have performed a self-review of my own code
• I have commented my code where relevant, particularly in hard-to-understand areas
• My changes generate no new console warnings
• The base of this PR is master if hotfix, develop if not

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[evgenyboxer]

This will help the user understand where the amounts are coming from when exploring Gnosis explorer.