enable per-page caching on the public alerts page

By default the public alert page is cached 300s, but that can be changed with an environment variable when deployed.

Before:

(Note i set the cache control to be five seconds by default, for local development)

After:

I started to get really nervous looking at the number of O4 insignificant events, so let's store that page in cache longer.

approved this merge request

Looks good to me

merged

mentioned in issue #318 (closed)

mentioned in merge request !151 (merged)