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.
Add subtasking strings to locales.
Translations updates.
