Refactor navbar javascript

[richardebeling]

Proposal from #2099: Refactor the javascript code that makes the language and staff-mode buttons spin on user click

Original text:

How do you feel about removing the javascript for this and replacing everything inside the li with

<form id="set-lang-form" method="post" action="{% url 'evaluation:set_lang' %}">
    {% csrf_token %}

    <div class="btn-group">
        {% for language_code, language_name in languages %}
            <button
                type="submit" name="language" value="{{ language_code }}"
                class="btn btn-sm{% if current_language == language_code %} active{% endif %}"
                data-bs-toggle="tooltip" data-bs-placement="bottom" title="{{ language_name }}"
            >
                <span>{{ language_code|upper }}</span>
            </button>
        {% endfor %}
    </div>
</form>

<script type="text/javascript" defer nonce="{{ request.csp_nonce }}">
    document.getElementById("set-lang-form").addEventListener("submit", event => {
        setSpinnerIcon(event.submitter.querySelector("span"));
    });
</script>

This also needs a change in setSpinnerIcon to something like

function setSpinnerIcon(element) {
    if (typeof element === "string" || element instanceof String)
        element = document.getElementById(element);
    element.innerHTML = "";
    element.className = "fas fa-spinner fa-spin";  // clears all other classes
}

Originally posted by @niklasmohrin in #2099 (comment)