Fix internal links so that they work with latest ReportLab

[akrabat]

Prior to ReportLab 3.5.19, a link that wasn't well-formed was assumed to
be an internal anchor. This was fixed in 3.5.19, so we now need to
prefix all internal links we generate with #.

This fixes #772

To Test:

Given test.rst:

===============================
Document with Table of Contents
===============================


.. contents:: Table of Contents
   :depth: 2

Heading 1
=========

Normal, **Bold**, *Italic*.

This is a paragraph in Page 1 [#]_

.. [#] This is a footnote in page 1

Run:

rst2pdf test.rst

Open the resulting test.pdf and resize into a vertically narrow window and ensure that

• The TOC link takes you to the Heading 1 section
• The footnote at the end of the paragraph takes you to the footnote
• The link in the footnote takes you back the paragraph

Test with ReportLab 3.5.12 and then update ReportLab to the latest, rebuild and test again.