Configurable validity for each announcement

[mahendrapaipuri]

Hello,

Firstly, thanks a lot for this project. It is quite useful and we are using it in our JupyterHub deployment.

In a typical scenario where admins would like to announce, say maintenance of JupyterHub, after the maintenance, ideally we would like to have announcement disappeared. Currently, we will have to add an empty announcement to make it disappear (I guess?). How about adding a validity for each announcement and the announcement will be automatically purged after it reaches its validity. If admin does not provide a validity for a given announcement, we use the default lifetime_days to get a default validity?

We did something like this for our deployment. Is it something you would be interested to add to the project? If so, I can put up a PR.

Cheers!

[rcthomas]

Sure, if you're willing to put together a PR, I'll take a look, thanks!