Mejai gives you cheap AP that can build and if fully stacked (which shouldn't be too hard given how many assists Janna can get) Janna can turn into a strong AP force rather than just a good support.
Boots of Mobility help you get to the team fight faster and with Shurelya's active and CDR, you can do without Ionian Boots.
Shurelya's, Aegis, and Solari help with team survivability for whole team and Janna herself.
Why abyssal scepter? The aura helps lower magic resist of enemy team while giving Janna a flat chunk of AP in case Mejai's has not been working out as well as you hoped.

*note:
-Zhonya's Hourglass is a good item to combo with your ult or just to decoy during team fights if you are getting singled out.
-always get wards and Oracle; be a good supportf
-Janna can go AD. Crazy to think but all of her abilities are meant for effect more than damage, so why not get crazy and make her an AD: she has a knock up, slow, and a shield that gives bonus damage.