fix: inputs don't provide styling for custom components.

[bundyo]

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What is the current behavior?

Custom components based on TextField can't be styled as a TextField easily.

What is the new behavior?

.nt-input class to style them.

Fixes #261.

BREAKING CHANGES:

• .nt-input class is now used for both inputs and input groups. The group top/bottom margin is removed and the whole group has the same margin as the TextField. The overall look is the same. Low impact - users most likely don't rely on the group margin at all.