move default modules from /modules/default to /defaultmodules (#4019)

Since the project's inception, I've missed a clear separation between
default and third-party modules.

This increases complexity within the project (exclude `modules`, but not
`modules/default`), but the mixed use is particularly problematic in
Docker setups.

Therefore, with this pull request, I'm moving the default modules to a
different directory.

~~I've chosen `default/modules`, but I'm not bothered about it;
`defaultmodules` or something similar would work just as well.~~

Changed to `defaultmodules`.

Let me know if there's a majority in favor of this change.

defaultmodules/alert/templates/alert.njk

@@ -0,0 +1,20 @@
+{% if imageUrl or imageFA %}
+  {% set imageHeight = imageHeight if imageHeight else "80px" %}
+  {% if imageUrl %}
+    <img src="{{ imageUrl }}" height="{{ imageHeight }}" style="margin-bottom: 10px" />
+  {% else %}
+    <span
+      class="bright fas fa-{{ imageFA }}"
+      style="margin-bottom: 10px;
+                     font-size: {{ imageHeight }}"
+    ></span>
+  {% endif %}
+  <br />
+{% endif %}
+{% if title %}
+  <span class="thin dimmed medium">{{ title if titleType == 'text' else title | safe }}</span>
+{% endif %}
+{% if message %}
+  {% if title %}<br />{% endif %}
+  <span class="light bright small">{{ message if messageType == 'text' else message | safe }}</span>
+{% endif %}

defaultmodules/alert/templates/notification.njk

@@ -0,0 +1,7 @@
+{% if title %}
+  <span class="thin dimmed medium">{{ title if titleType == 'text' else title | safe }}</span>
+{% endif %}
+{% if message %}
+  {% if title %}<br />{% endif %}
+  <span class="light bright small">{{ message if messageType == 'text' else message | safe }}</span>
+{% endif %}