Fix height / overflow behaviour of add token list

[danjm]

This PR fixes the problem with the height / overflow of the add token lists that causes the page footer buttons to be hidden by the scroll. This can be seen below:

By setting min-height: 0; on the parent div of the overflowing elements, the problem is fixed (for reasons explained here: https://stackoverflow.com/a/26916542/4727685). The fix can be seen below:

The changed css rule is only used on the add token screen and in the gas modal. The gas modal remains unchanged after this change.

[metamaskbot]

Builds ready [c913648]

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

Page Load Metrics (751 ± 79 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	30	124	60	26	12
		domContentLoaded	561	1050	749	165	79
		load	563	1052	751	165	79
		domInteractive	561	1049	749	165	79