Implement inline conditional text

- support conditional text depending on ouptut for inline text
- only support HTML, PDF and EPUB outputs
- insert different replace directives in conf.py at build time via sed scripts
- edit workaround for EPUB Sphinx bug
- create sed scripts to insert different replace directives in conf.py at build
  time
- put some terms in bold only in HTML version for SEO
- remove conditional substitutions from conf.py

Signed-off-by: Olivier Carrère <carrereo@gmail.com>
Acked-by: Angela Cristina <acristina@redaction-technique.org>