Add ability to ask external caches to invalidate.

[dracos]

Add a cached_urls method to the Comment, FoiAttachment, InfoRequest, PublicBody and User models, containing the entries that those models wish to invalidate when an event is logged via log_event or when the censor rules change. Entries are either a fixed path to be purged or a regex beginning with '^' to be banned.

Adds a NotifyCacheJob that for each of the object's cached_urls, for each locale, for each host in VARNISH_HOSTS, makes BAN or PURGE HTTP requests to the host, to invalidate the relevant entries.

[garethrees]

I expect you're aware but if not FYI Alaveteli has some prior art of this: #4252, #4232

[dracos]

Hi @garethrees, yes, thanks, I've put all the detailed discussion on https://github.com/mysociety/sysadmin/issues/1824

[gbp]

This is looking good to me. There are more I could do to this but for now I have committed some fixups to do things is slightly better Ruby approach but functionally this should be the same. Many thanks @dracos

Changes:

1. remove multiple conditionals, in favour for a callback when enqueuing the job
2. move post event logging code into InfoRequestEvent after create callback, and added specs
3. refactored job, removing heavily nested code and improve success/failure logging.

[dracos]

Thanks - that callback is definitely nicer :) Only query I have is can Net::HTTP hold open a connection - I had it outside the URLs loop in the hope it would use the one connection for all of them, but probably makes no difference.

[gbp]

Only query I have is can Net::HTTP hold open a connection

Ah, as it is then no it doesn't. We can close the connection manually though if we don't use a do/end block. I'll push another fixup.