fr: Updating French translation file, and a few messages.

[yohcop]

Re-generated a new po/messages.pot, and merged into po/fr.po.

Updated a handful of message translations as well, but this small PR is mostly to 'get my feet wet' with the whole process.

Part of #323.

[mgeisler]

Thanks @yohcop! I added a reference to the French translation issue (that way the PR shows up on the issue).

[yohcop]

I'm not sure if I did this right. The file is so big, as most line numbers have changed, that github doesn't want to load the diff. Let me know if I should do things differently.

[sakex]

This is a huge CL, we can merge it like this for now. I will have a longer look on this next week

[sakex]

Also, it doesn't let me add my review comments for some reason

[mgeisler]

I'm not sure if I did this right. The file is so big, as most line numbers have changed, that github doesn't want to load the diff. Let me know if I should do things differently.

You did it right! The problem is that msgmerge formats the PO file differently than our mdbook-xgettext plugin. I should fix that so I filed google/mdbook-i18n-helpers#32 for it.

[mgeisler]

I merged this, thanks for looking it over!

@sakex, you should generally merge PRs when approving them since most people won't have write access to the repository.