Fixed closing tags in syntax highlighting tests.
Add background color to line numbers of highlighted code.
Replace the hardcoded "CodeRay" css class name for highlighted elements.
Extract CodeRay calls to Redmine::SyntaxHighlighting (#2985).
Reverts r1476 that is no longer needed after r3582.
Do not html escape code that is going to be highlighted (#2985, #3359).
Optimization: load attachments when needed.
Enlarges the sidebar.
Move issue watchers to the sidebar.
Adds index on issues nested set columns.
