Make language of membership summary consistent

fixes #401
add TODO on adding class
remove member title from page title

fixes #407
Only add "lead" class to first instance of paragraph
Don't say "Senator for NSW, in the Senate"
Refactor common code
Use helper method
Say "member since" for current Members or Senators
Duplicate code since these feel different enough now
Extract logic into helper method
Refactor out variables
Merge pull request #409 from openaustralia/issue401

Fixes the descriptions of previous membership