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.
Slight UI changes to the subtasks tree.
