Make distinction for disabled text field in membership step

Right now, on the Create Flavor modal > Flavor Access tab,
you see two text fields.  Whether on not they are disabled, they
look the same unless you hover over it (to see a red cancel
cursor).

This patch shows an immediate visual distinction between
disabled/enabled text fields for membership text fields
(also used in Create Project modal > Project Members).

Change-Id: I15c83ff2cf2fb125dfbf368d2a2be637ab69dab0
Closes-Bug: #1325666
Merge "Make distinction for disabled text field in membership step"