Tock Holiday Banners

[cantsin]

From https://github.com/18F/TLC-crew/issues/653:

Back in the day we used to have banners in tock that reminded us to bill OOO for holidays. I found them to be helpful.

Let's figure out ownership and document this process somewhere (if not already).

[kfoley-18F]

Banners are a feature that exist within Tock and just require someone to curate them.
I'm happy to walk folks through the process for updating and curating Tock banners.
(this is one part of the broader Tock SO handoff that needs to happen, and I'm waiting for @hbillings to return to participate)