Remove the mix between XHTML syntax and HTML

There is a weird mix between XHTML and HTML syntax in phpBB pages.

Sometimes, the void elements end by /> (XHTML syntax).

Some examples:

• Void elements inserted in the prosilver template files
• Avatars (XHTML syntax is inserted by the function phpbb_get_avatar)
• S_SEARCH_HIDDEN_FIELDS, other HIDDEN_FIELDS, META (refresh in viewonline), or <br /> of RANK_IMG inserted in memberlist_body

In contrast, other void elements end by > (HTML syntax), at least inside the posts content.

• <br>
• emojis and smilies
• images added by the  [*img*] tag (NOT the attached images, which end by />). Oddly, in the bbcode.html file, images code end by /> too

The code in phpBB board would be more consistent by using only XHTML or HTML syntax, instead of mixing both.