Set up JavaScript testing framework and write initial tests

[tobyzerner]

Now that we're using Mithril, we have complete freedom over what we use and how we use it.

A good starting point might be to copy whatever Mithril does itself.

[tobyzerner]

OK I made a start on this too (at least I fixed JSHint errors) but I'm struggling to even get the first test running. All index-test.js is doing is calling visit('/') but it is still failing with the following unhelpful message:

    ---
        actual: >
            null
        message: >
            Error: Something went wrong
        Log: >
    ...

Can anyone lend a hand?

[tobyzerner]

Sorted it out — it was because the API URL was wrong. Fixed in d13ef6f. Wrote some initial tests in bbf2c97. One is failing, waiting on kurko/ember-json-api#59

Ultimately we'll need to set up fixtures/synthetic AJAX responses rather than the calling a live API.

[tobyzerner]

Closing in favour of flarum/issue-archive#424