The typeahead suggests a human-readable `pretty_name` for the
`language` field in the add-playgrounds form.
The suggestions are sorted based on the popularity of these
languages.
E.g: A `py` prefix would match with `Python` first before
matching with others like `Python 2.x` based on priority.
The design of the form is similar to the linkifiers page
and is styled similarly.
The introduction text for "Code playgrounds" is improved
with more details and examples.
Also, we can remove the hardcoded playground and the fix
we had previously done to prevent breaking the hardcoded
playground.
To prevent breaking of the hardcoded playgrounds, we resort
to checking if realm_playgrounds is empty and falling back
to the hard-coded list if so. This logic is removed in the
followup commit which introduces the UI to add a playground.
