Allow templates to be overridden in data/templates/

I'd like to customize my instance's theme beyond what's possible with
_theme.scss.  This patch would allow me to do that, and keep my changes
self-contained in data/ without maintaining a local patchset over
app/templates/.

For utils.html, I've also added scoped blocks around the body of every
macro.  This allows the macros to be overridden individually in
data/templates/utils.html, without copying the whole file.  For example,
to only override the display of a specific actor's name/icon:

    {% extends "app/utils.html" %}
    {% block display_actor %}
    {% if actor.ap_id == "https://me.example.com" %}
    <!-- custom actor display -->
    {% else %}
    {{ super() }}
    {% endif %}
    {% endblock %}

data/templates/app

@@ -0,0 +1 @@
+../../app/templates/