Show transaction fee units on approve screen

[Gudahtt]

The units for the amounts shown on the approve screen in the transaction fee section were missing. It appears that they were present in an early version of the approve screen (#7271) but they got lost somewhere along the way.

Before:

After:

[whymarrh]

LGTM

[metamaskbot]

Builds ready [1b3f194]

• builds: chrome, firefox, opera, edge
• bundle viz: background, ui, inpage, contentscript, ui-libs, bg-libs, phishing-detect
• dep viz: background
• all artifacts

[Gudahtt]

This should be ready to review again. I had to update one test.

This is just a temporary solution - it's still lacking in a few ways:

• We should be using metamask.nativeCurrency instead of hard-coding ETH
• We should be displaying either the fiat or native currency based upon the "Primary Currency" config setting
• We should be omitting the fiat currency on test networks if the user has that setting enabled

But in the interest of getting this out there sooner, I think we should go with this for now, and at some point refactor the CurrencyDisplay component to be a bit easier to use for cases like this. Then ideally we'd use the CurrencyDisplay component here and it'd take care of those details for us.

[metamaskbot]

Builds ready [67a149d]

• builds: chrome, firefox, opera, edge
• bundle viz: background, ui, inpage, contentscript, ui-libs, bg-libs, phishing-detect
• dep viz: background
• all artifacts

[whymarrh]

I agree that this is a good quick fix