WIP: Improve French translations

WIP : The file has not been tested yet. I also will ask for feedback from other Pleroma users, as well as from the previous translator if possible.

I edited quite a few of the strings for one or more of the following reasons:

• consistency
• awkward wording
• too long to fit in the UI
• grammar
• typos

Some translations I did not change, but might be worth reconsidering:

• "journal" for "timeline"
• "partager" for "repeat"