Because the token values escaped by BoldKeywordFilter are simply
intermediate values and are not directly included in HTML templates,
use Python's html.escape() instead of django.utils.html.escape() to
eliminate the overhead of converting the token values to SafeString.

Also pass quote=False when calling escape() since the token values will
not be used in quoted attributes.

sqlparse's SerializerUnicode filter does a bunch of fancy whitespace
processing which isn't needed because the resulting string will just be
inserted into HTML.  Replace with a simple EscapedStringSerializer that
does nothing but convert the Statement to a properly-escaped string.

In the process stop the escaping within BoldKeywordFilter to have a
cleaner separation of concerns:  BoldKeywordFilter now only handles
marking up keywords as bold, while escaping is explicitly handled by the
EscapedStringSerializer.

Instead of using a regex to elide the select list in the simplified
representation of an SQL query, use an sqlparse filter to elide the
select list as a preprocessing step.  The result ends up being about 10%
faster.

Instead of only eliding select lists longer than 12 characters, now only
elide select lists that contain a dot (from a column expression like
`table_name`.`column_name`).  The motivation for this is that as of
Django 1.10, using .count() on a queryset generates
    SELECT COUNT(*) AS `__count` FROM ...
instead of
    SELECT COUNT(*) FROM ...
queries.  This change prevents the new form from being elided.

If a query has subselects in its WHERE clause, do not elide the select
lists in those subselects.

The "<strong>" tokens inserted by the BoldKeywordFilter were causing the
AlignedIndentFilter to apply excessive indentation to queries which used
CASE statements.  Fix by rewriting BoldIndentFilter as a statement
filter rather than a preprocess filter, and applying after
AlignedIndentFilter.

When formatting SQL statements using sqparse, grouping only affects the
output when AlignedIndentFilter is applied.

By using a settings_changed signal receiver to clear the query caching,
the parse_sql() and _parse_sql() functions can be merged and the check
for the "PRETTIFY_SQL" setting can be moved back inside the
get_filter_stack() function.