[IMPR] move character bases functions from page to tools.chars

unicode2html and url2unicode belongs to page titles but is also
independed from other framework parts. Therefore move them to the
module dealing with characters and strings. The page module is too
big and complex already.

- move page.UnicodeToAsciiHtml to tools.chars.string_to_ascii_html
- move page.unicode2html to tools.chars.string2html
- move page.url2unicode to tools.chars.url2string
- deprecate page functions and their direct import from pywikibot
- deprecate pywikibot.site.BaseSite as encodings parameter in url2unicode
  function; BaseSite.encodings should be used instead
- update usage of unicode2html in reflinks.py
- update usage of  url2unicode in cosmetic_changes.py

Change-Id: Ifab5ab13bf18a2693c014b3d0beeee59e36e0c2a