This commit introduces rails system tests run with chromedriver, selenium,
and headless chrome to our testing toolbox.

We use the webdrivers gem and selenium-webdriver which is what
the latest Rails uses so the tests run locally and in CI out of the box.

You can use SELENIUM_VERBOSE_DRIVER_LOGS=1 to show extra
verbose logs of what selenium is doing to communicate with the system
tests.

By default JS logs are verbose so errors from JS are shown when
running system tests, you can disable this with
SELENIUM_DISABLE_VERBOSE_JS_LOGS=1

You can use SELENIUM_HEADLESS=0 to run the system
tests inside a chrome browser instead of headless, which can be useful to debug things
and see what the spec sees. See note above about bin/ember-cli to avoid
surprises.

I have modified bin/turbo_rspec to exclude spec/system by default,
support for parallel system specs is a little shaky right now and we don't
want them slowing down the turbo by default either.

PageObjects and System Tests

To make querying and inspecting parts of the page easier
and more reusable inbetween system tests, we are using the
concept of PageObjects in
our system tests. A "Page" here is generally corresponds to
an overarching ember route, e.g. "Topic" for /t/324345/some-topic,
and this contains logic for querying components within the topic
such as "Posts".

I have also split "Modals" into their own entity. Further down the
line we may want to explore creating independent "Component"
contexts.

Capybara DSL should be included in each PageObject class,
reference for this can be found at https://rubydoc.info/github/teamcapybara/capybara/master#the-dsl

For system tests, since they are so slow, we want to focus on
the "happy path" and not do every different possible context
and branch check using them. They are meant to be overarching
tests that check a number of things are correct using the full stack
from JS and ember to rails to ruby and then the database.

CI Setup

Whenever a system spec fails, a screenshot
is taken and a build artifact is produced after the entire CI run is complete,
which can be downloaded from the Actions UI in the repo.

Most importantly, a step to build the Ember app using Ember CLI
is needed, otherwise the JS assets cannot be found by capybara:

- name: Build Ember CLI
  run: bin/ember-cli --build

A new --build argument has been added to bin/ember-cli for this
case, which is not needed locally if you already have the discourse
rails server running via bin/ember-cli -u since the whole server is built and
set up by default.