emacs: get rid of trailing spaces in notmuch-hello view

This patch removes trailing spaces in notmuch-hello view.

A side effect of this change is that tag/query buttons no longer
include a space at the end.  This means that pressing RET when the
point is at the first character after the tag/query button no longer
works (note that this is the standard behavior for buttons).  We may
change this behavior in the future (without adding trailing spaces
back) if people would find this change inconvenient.