v2.6.4-rc1 for phpBB v3.3.0

A maintenance release with minor improvements including:

• Minor corrections resulting from a code review of an earlier version
• Updates from the v2.6.3 release (which did not come directly from this repository)
• Replace topic (etc) icons with new versions based on prosilver version but with blue borders changed to grey
• Remove stylesheet.css and move contents to overall_header.html
• Add colours.css so that all items in prosilver template files render with correct colours
• Move colour definitions for classes inherited from prosilver to colours.css
• Minor modifications to the header class to improve responsive layout
• Improve option for removing grey side borders from header image
• Improve layout of smileys