Remove <br>s for HTML5 galleries

[obenland]

HTML5 support for galleries was introduced recently (r27302). Shortly after that, we decided to seize the opportunity and disable default gallery styles, when a theme supports HTML5 galleries (r27396).

Last week, Nacin wondered if it would make sense to also remove the <br> element, that gets inserted to force line breaks. After we weighed pros and cons I asked some colleagues for their opinion, and they unanimously agreed that we should remove them. So let's do it and improve the markup even further.

[obenland]

Includes support for 2013 and 2014

[nacin]

In 27914:

Remove <br> elements for HTML5 galleries.

props obenland.
fixes #27637, see #26697.

[obenland]

Twenty Thirteen and Fourteen need a little attention on this.

[lancewillett]

In 27926:

Twenty Thirteen and Fourteen: style to support the new HTML5 line-break-less galleries. Props obenland, see #27637.