[swaps] ERC20 custom approvals not human readable

Describe the bug
Approvals in swaps are in scientific notation instead of decimal format.

Steps to reproduce (REQUIRED)
Steps to reproduce the behavior, libraries used with version number, and/or any setup information to easily reproduce:

1. Perform a swap, and edit the spend allowance for a token (the approve method)
2. Try changing the allowance

Expected behavior
A clear description of what you expected to happen.
The output for the user should not be in scientific notation, instead it should be represented in a similar fashion that the token is represented in the assets tab.

Screenshots
If applicable, add screenshots to help explain your problem.

Browser details (please complete the following information):

• OS: [e.g. OS X, Windows] OS X
• Browser: [e.g. Chrome Version 79.0.3945.79 (Official Build) (64-bit), Firefox Browser 71.0 (64-bit)] Firefox
• MetaMask Version: [e.g. 5.0.2 - find it in Settings > About] 10.5.1

Edit 1: updated bug description & removed additional information

Start by reproducing the issue in the swaps flow: perform a swap, edit the token spend allowance through the approve method, and change the allowance. Trace the UI entry point for editing approvals and compare its formatting with the token display in the assets tab. Done means the allowance is shown in decimal format rather than scientific notation.