Fix varius db_prefix issues (#1489839)
- Move DDL script execution code to rcube_db class(es).
- Improve prefix replacement code, so index names are also modified
Fix too small length of users.preferences column data type on MySQL
Fix redundant warning when switching from html to text in empty editor (#1489819)
We also skip ajax request in such a case. We assume "empty" here means "with no text excluding whitespace".

Conflicts:

	program/js/app.js
Fix invalid host validation on login (#1489841)
Support 'error' and 'body_file' return attribs in 'message_before_send' hook (#1489595)

Conflicts:

	CHANGELOG
Add changelog line removed by mistake in last commit
Replace setSelectionRange() call with universal rcmail.set_caret_pos()
Fix unwanted horizontal scrollbar in spellcheck languages list in Chrome browser