Deprecate the use of <a> for .contentItemLink

[dtdesign]

The new .contentItemList was introduced as a generic implementation for content pieces that rely on teaser images. However, the DOM is somewhat flawed by expecting a link to wrap around the whole content section, which easily collides with certain content elments.

<a href="…" class="contentItemLink">
  <!-- content -->
</a>

Should be changed into:

<div class="contentItemLink">
  <!-- content -->
  <a href="…" class="contentItemLinkShadow"></a>
</div>

The CSS remains unaffected, causing no visual change if the old DOM is continued to be used, preserving compatibility with existing implementations.

We also need to deprecate the changes related to the removal of links in the message parsers. They tend to be unreliable and require way more changes than anticipated.