Merge branch 'save-button-template' into 'master'

Make save_button template customizable with parameters.

See merge request AlekSIS/AlekSIS!133

aleksis/core/templates/core/save_button.html

 {% load i18n %}
 <button type="submit" class="btn waves-effect waves-light green">
-  <i class="material-icons left">save</i> {% trans "Save" %}
+  {% trans "Save" as default %}
+  <i class="material-icons left">{{ icon|default:"save" }}</i> {{ caption|default:default }}
 </button>