This commit refactors the <input> and <button> used to validate the MSS
key to its own components. After doing this, we will be able to reuse
this components outside of the settings app. In particular, we want to
be able to use it in the welcome wizard app.
It also implements one small change to the verify button that is part of
the changes related to the welcome wizard. Now the verify button is
disabled when the MSS key <input> is empty.
[MAILPOET-4818]
