emacs: fix `notmuch-wash-region-to-button' to work at beginning of buffer

`Notmuch-wash-region-to-button' is the function that creates hidden
regions with buttons for signatures, citations and original messages.
Before the change, it did not work correctly if the to-be-hidden
region started at the beginning of a message: the visibility toggle
button was hidden as well.  The patch fixes this.  There are two parts
in the fix:

* Use `insert-before-markers' instead of `insert' for creating the
  button, so that it does not get added to the hidden overlay.

* Stop using PREFIX argument for adding a newline before the button.
  The newline should not be added before a button at the beginning of
  buffer.

The corresponding test is fixed now.