Our theme system is very complex and it can take a while to figure out how to invalidate the various types of caches that are used throughout the theme system. So, having a single helper method that invalidates everything can be useful in emergency situations where there is no time to read through the code and figure out how to clear the various caches.
Internal ticket: t64732.
`TestLogger` was responsible for some flaky specs runs:
```
Error during failsafe response: undefined method `debug' for #<TestLogger:0x0000556c4b942cf0 @warnings=1>
Did you mean? debugger
```
This commit also cleans up other uses of `FakeLogger`
Discourse users and associated accounts are created or updated when a
user logins or connects the account using their account preferences.
This new API can be used to create associated accounts and users too,
if necessary.
It's very easy to forget to add `require 'rails_helper'` at the top of every core/plugin spec file, and omissions can cause some very confusing/sporadic errors.
By setting this flag in `.rspec`, we can remove the need for `require 'rails_helper'` entirely.
