emacs: break up notmuch-show-archive-thread-internal into two more generally useful...

Break up notmuch-show-archive-thread-internal into two new functions:

notmuch-show-tag-thread-internal: applies a tag to all messages in
thread.  If option remove flag is t, tags will be removed instead of
added.

notmuch-show-next-thread: moves to the next thread in the search
result.  If given a prefix, will show the next result, otherwise will
just move to it in the search view.

Two new interactive functions, notmuch-show-{add,remove}-tag-thread,
are also added.  Together, these provide a better suit of thread
tagging and navigation tools.

The higher level thread archiving functions are modified to use these
new function.