Removed duplicate caching of mysql_version.

The manual caching in self.server_version and the cached_property
decorator are redundant.
Implemented PostgreSQL version as a cached property.
Added required methods in BaseDatabaseWrapper.

I should have included this in 29628e0b6e5b1c6324e0c06cc56a49a5aa0747e0.
Fixed #19837 -- Refactored split_exclude() join generation

The refactoring mainly concentrates on making sure the inner and outer
query agree about the split position. The split position is where the
multijoin happens, and thus the split position also determines the
columns used in the "WHERE col1 IN (SELECT col2 from ...)" condition.

This commit fixes a regression caused by #10790 and commit
69597e5bcc89aadafd1b76abf7efab30ee0b8b1a. The regression was caused
by wrong cols in the split position.
Fixed #10870 -- Added aggreation + generic reverse relation test
Removed accidentally committed file
Avoided related_name conflicts in tests
Update docs/topics/class-based-views/generic-display.txt

simple typo in "Making friendly template contexts"
Implemented Oracle version as a cached property.
Ensured a connection is established when checking the database version.

Fixed a test broken by 21765c0a. Refs #18135.
Fixed #18789 - Fixed some text wrap issues with methods in the docs.

Thanks neixetis@ for the report.
Updated a couple admonitions to use the warning directive.
Fixed #19852 - Updated admin fieldset example for consistency.

Thanks chris.freeman.pdx@ for the suggestion.
Added a new required ALLOWED_HOSTS setting for HTTP host header validation.

This is a security fix; disclosure and advisory coming shortly.
Restrict the XML deserializer to prevent network and entity-expansion DoS attacks.

This is a security fix. Disclosure and advisory coming shortly.
Checked object permissions on admin history view.

This is a security fix. Disclosure and advisory coming shortly.

Patch by Russell Keith-Magee.
Added a default limit to the maximum number of forms in a formset.

This is a security fix. Disclosure and advisory coming shortly.
Update 1.5 release notes for XML and formset fixes.
Merge pull request #737 from hrbonz/docs_CBV_generic_views_typo

Fixed typo in docs/topics/class-based-views/generic-display.txt
Fixes typo introduced by django/django@08dc90bccf7c4ffa8b04064d74b54c1150af5ff9. This is described in Trac ticket:19855.