make displaying tags line in status buffers a choice

Move the code that collects information about the two closest tags and
displays them info new function `magit-insert-status-tags-line' and add
new option `magit-status-insert-tags-line' to control whether that
function should actually do anything or return immediately.
add option to choose whether HEAD/tag is subject in tags line

Add new option `magit-status-tags-line-subject' which controls whether
each tag or HEAD is considered the subject in the two imaginary
sentences the tags line in status buffers imply.
Merge branch 'feature/improve-tags-line'