Switch from php templating to Twig

[hemberger]

We currently use PHP templating to render the display pages. We may want to consider incrementally migrating to Twig (https://twig.symfony.com/doc/) instead.

Benefits:

• One of the most standard PHP templating libraries.
• Enforces a serializable, read-only data model (whereas PHP templating still has access to our PHP libraries and can modify the state of existing objects and databases).
• The syntax is less verbose {{ bla }} vs. <?php echo $bla; ?> and has many convenient functions for manipulating the data (e.g. escaping etc.).

Downsides:

• It's an entirely new language that must be learned to maintain our codebase.
• It creates a dependency on an external library (though it is so popular that it is unlikely to be abandoned).
• The additional overhead likely would incur a small runtime penalty.