hgweb: replace implicit <tbody> with explicit <thead> where appropriate
Some templates in paper style use <tbody> elements inside <table> to assign a
class to "body" part of that table (in this case, to make rows striped). The
problem is that the <tbody> is preceded by <tr> element, which browsers
understand as an implicit start of table body, so the following exlicit <tbody>
will actually be "nested", which is not valid.
Since that first <tr> contains table headers, wrapping it in <thead> is both
semantically correct and follows the advertised XHTML 1.1 doctype.