Allow UfAlert style customization

[lcharette]

Currently the alerts are rendered by uf-alerts.js, which doesn't allow style customization through a sprinkle and/or Twig template. A Twig template file should be used to define the html code of the full alert. The alert should then be rendered using Handlebar.js or a rendered version should be loaded via Ajax instead of loading only the son data.

Fixing this should probably fix #621 at the same time.

[alexweissman]

Or, we can just have a Handlebars template in the DOM (wrapped in a <script type="text/x-handlebars-template"> block), and then pass that to uf-alerts. This is basically how things work for uf-table.

For example, the Handlebars template for the "user info" cell of the users table:

    <script id="user-table-column-info" type="text/x-handlebars-template">
        <td data-text="{{row.last_name}}">
            <strong>
                <a href="{{site.uri.public}}/admin/users/u/{{row.user_name}}">{{row.first_name}} {{row.last_name}} ({{row.user_name}})</a>
            </strong>
            <div class="js-copy-container">
                <span class="js-copy-target">{{row.email}}</span>
                <button class="btn btn-xs js-copy-trigger"><i class="fa fa-copy"></i></button>
            </div>
        </td>
    </script>

This is a Handlebars template, so ufTable extracts it from the DOM and then uses it to generate the table cells.

[lcharette]

I looked at Handlebar doc and I guess it would be the easiest way to do this. The actual Handlebar template should be in a separate html.twig file and included in the DOM so it's easy for a Sprinkle to overwrite that template.

At the same time, I was thinking about splitting multiple alert into different alert box instead of a <ul> list. The "dismiss" button and even icon could be added too.

I'll assign this to myself since I do want to try out and learn how Handlebar work. Might be useful in my own project ;)

[lcharette]

This is implemented in commit above. Alerts style can be customized by overwriting app/sprinkles/core/assets/local/core/css/uf-alerts.css and/or app/sprinkles/core/templates/components/alerts.html.twig in a sprinkle.