Move cache initialisation under supervision trees

[geekingfrog]

This PR moves the caching logic into logically independent modules, and define supervision tree to start the caches and do initialisation at the same time.

The main benefit is defining a cache and warming it up is now in one place instead of spread across the application.ex and startup.ex files.

One thing to note: some caches are used together. For example config_site_type_store and :config_site_cache are both used by add_site_config_type. In this case, the restart strategy should be :one_for_all so that if a cache dies, the other is also terminated, and then both are restarted before initialisation.

This should be reviewed commit by commit where I tried to only move small bits at a time.

Ultimately I want to straighten up the startup logic which has a bunch of init code outside supervision trees, but that'll come later.

How to test that?

There shouldn't be any functional changes, so:

• starting up
• login as admin and doing a quick permission check (can you access protected parts?)
• start up a lobby and issue a command like $help or $whois

[Teifion]

This is a really sensible idea; I'll be implementing it in my other projects. Thank you :)

[L-e-x-o-n]

Works on integration server.