Ad margins need flexibility

[iMattPro]

The 10px margins added to our ads need to be more flexible.

We should separate the margin into a separate class.

The reason is so, for example individual template locations, or other extensions extending our locations, can choose if they want the margins or not.

e.g.:

<div class="ad-center ad-margin">{{ AD_BEFORE_HEADER }}</div>

.ad-center {
	display: flex;
	justify-content: center;
}

.ad-margin {
	margin: 10px 0;
}

[iMattPro]

It may actually even be better to not have a fixed margin class, and instead use inline style="" in each template event so each ad location can have truly its own unqiue margin/styling needs.

[iMattPro]

We should also rename our styling classes.

I propose phpbb-admanagement-center for 2 reasons:

1. Namespacing it with our vendor ext name will ensure no conflicts with other classes from extensions
2. Amazingly, the class name ad-center is picked up by my ad blocker and hides the content. We should try to minimize this problem, for example I was using an ad location just for a simple text only link back to my hosting provider...something that is not an advertisement at all...and my ad blocker is blocking it because of this class when named ad-center.