[warnings] put an end to warnings in the sqlite driver over `str` being sent instead of unicode strings

[web/request] clearly mark user_callback-related methods as deprecated The deprecated decorator not only emits a warning at run-time but above all makes the deprecation immediately apparent in the source. closes #3567793.

[book] document __unique_together__, remove bad RQLUniqueConstraint example RQLUniqueConstraint should be avoided whenever possible. Related to #3753250.

[schema] stop using RQLUniqueConstraint (closes #3753250) The last uses are replaced with unique together constraints.

[dataimport] Have ucsvreader's API match that of csv.reader (closes #3705701) 'separator' and 'quote' are replaced with 'delimiter' and 'quotechar'.

[utils] Add a '_cwtracehtml' GET parameter to trace self._cw.w() calls (closes #4601327) The core of this patch is in UStringIO.write(). When tracing is enabled, write() doesn't just append the 'value' argument to the underlying list. Instead, a stack trace is recorded and a special HTML "source" is formatted. The output with tracing enabled is an HTML page, with the original HTML escaped, and made clickable to show the stack trace when the write() call was done. This allows answering the recurring question: "who wrote this tag here?!"

[datafeed] Add a raise_on_error parameter to DataFeedSource.extid2entity And pass the option from various `process_` methods in existing parsers. This makes debugging in tests easier. Closes #4601191.

[test/view forms] use the official, undeprecated API