Align basic-topic-list table structure with Categories page topic-list table structure

This is an optimization for people that choose their table row highlighting to be inverse of the default Discourse theme.
If people choose to have the heading row (or individual th columns) and 'even' rows highlighted in topic-list.scss for the Categories page, the alternate coloring is not carried over to the User Activity page's Messages list. Because the basic-topic-list table there doesn't use the THEAD and TBODY wrappers, the first data row is effectively an 'odd' row, so highlighting is now applied both on the heading row and the first data row.
By adding the THEAD and TBODY wrappers, we make sure that the table structures across the app are the same and global row highlighting styles work out of the box also on the User Activity page.
SECURITY: sanitize markdown urls (prevent XSS)
fix the build
Merge pull request #2198 from borhub/prevent_bgcolor_issues_in_basic_topic_list

Align basic-topic-list table structure with Categories page topic-list table structure