Fix page titles containing UTF-8 characters.

Thanks to Daniel Berteaud.
Update fr_FR locale.

- Some misspelling
- Some comments (between <em> tags) were misplaced
- There was no difference between uploads and downloads (both were translated
  as "Téléchargement"). I changed this so that uploads are now translated as
  "Envoi"
- Grants were translated as "Concessions", which is a valid literal
  translation, but doesn't seems very natural in this case. I changed it as
  "Invitation". I think it makes more sens as when you create a grant, you
  invite someone to send you a file

Thanks to Daniel Berteaud.
Fix external HTTP auth on the REST interface when PHP_AUTH_PW is not set.

Some external authenticators might not provide a clear-text password. Only
compare the password if PHP_AUTH_PW exists, making it identical in behavior
with the normal web interface.