Support localization in Gatsby app

[obahareth]

Checklist:

• [x] Use path (e.g. /ar) to set current locale.
• [x] Set Arabic as the default locale.
• [x] Allow changing locales (change path via gatsby's navigate).
• [ ] Add UI to switch locales (and perhaps trigger a refresh when switched?).
• [x] Create localization files.
• [x] Move all static English "magic strings" into a localization file.
• [x] Translate English magic strings into an Arabic localization file.
• [x] Use some sort of framework or function to get strings for current locale.
• [x] Fetch all strings from locale files using framework/function mentioned above.

[obahareth]

I am actually going to start on this.

[obahareth]

I checked the stuff I finished so far, I still need help with the cookie/UI part. I already added react-cookie as a start.