Move alert to templatetag

One-sentence description

Why?

DRY, lots of alerts everywhere we could just easily make it a templatetag.

Current implementation

Lots of alerts, for example

<div class="alert alert-success alert-dismissable">
    {% trans "Your profile has been updated successfully." %}
    <button type="button" class="close" data-dismiss="alert" aria-label="Close">
        <span aria-hidden="true">&times;</span>
    </button>
</div>

Desired implementation

Put the html in an include and us a templatetag to make this easy to reuse. Could use https://django-bootstrap4.readthedocs.io/en/latest/templatetags.html#bootstrap4.templatetags.bootstrap4.bootstrap_alert, but not sure if we want to rely on this.

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