Added archive page & masthead template files, misc. changes / fixes

- Archive page templates for releases and videos. Initial HTML output
for releases (still working on videos)

- Masthead template files for area above content and sidebars

- Added custom slugs for releases and videos so the URLs look a little
cleaner when using custom permalinks

- Output release info in the Releases widget

- Updated code for Releases widget so that the widget is hidden on the
release archive page

- Added HTML output for Newsletter widget

- Miscellaneous back-end changes