Cast arguments for new Bignumbers() to string.

[danjm]

Due an unexpected issue in our current version of bignumber.js (MikeMcl/bignumber.js#148), new Bignumber(x) where x's significant digits were greater than 15 and x is not a string were throwing an error.

It seems that all our instances of new Bignumber(x) that were getting hit by numbers with that many significant digits were already casting args to strings, except for the new approve screen.

This PR updates all cases of new Bignumber(x) where x was not being converted to a string.

For a before and after test. Before:

• Run the app on develop, switch to mainnet and go to https://dai.makerdao.com/
• Click the 'Denied' toggle next to MKRWipe/Shut on the right side of the screen
• try to customize gas
• Gas will not update. check the console to see bignumber errors like that linked above

After:

• Run the app on this PRs branch and then follow the above steps
• Gas should correctly update
• submit the transaction (you can approve 0 MKR)
• the transaction should successfully confirm

[whymarrh]

Can we use the more explicit String(value) instead of the implicit string conversion?

[whymarrh]

LGTM

[danfinlay]

LGTM