fix(fr-FR.json): add missing translations specific to Mozilla.social

[MarcT0K]

This commit adds missing translations related features specific to Mozilla Social. I also fixed a few existing translations mentioning Elk instead of Mozilla.Social (translations based on en.json file).

I have two remarks about my translations:

1. There is a little ambiguity in the "invite" label. I don't know whether this label corresponds to the noun or the verb "invite". For now, my translation corresponds to the verb. I assumed this label could be used in a button. If not, the translation should be "Invitation".
2. I translated the message "New to Mozilla.Social?" by "Vous découvrez Mozilla.Social ?" (literally, "You are discovering Mozilla Social?") instead of "Nouveau·elle sur Mozilla.social ?", a more literal translation. Both versions are gender-neutral but the most literal translation is "harder" to read because of the word extension ensuring gender-neutrality (i.e., "·elle" after "Nouveau"). Hence, my version looks more natural because it does not use any gendered word.

NB: I am a native French speaker.