Fixing a spelling mistake [ci skip]

Fix #8414. Performance problem with postgresql adapter primary_key function.

Conflicts:
	guides/source/migrations.md

The release notes is changed to include only the major changes and point
to the changelogs for the complete list. This lets us talk more about the
features included in the upcoming release and make the guide more useful
and interesting than just a list of all changelogs.

This is still a WIP guide pending a lot of work.

[ci skip]

pg_namespace table isn't used.

This reverts commit 5ff59704.

Reason: Seems dubious and adds less value.

[ci skip]

be ignored. A regular expression constraint gets overwritten when the
routes.rb file is processed. Changed the overwriting to an ||= instead
of an = assignment.

Do not instantiate intermediate AR objects when eager loading.

Closes #3313

Locate `=require turbolinks` at the bottom of application.js

Turbolinks should be located at the bottom of application.js
to detect assets change properly.

Ensure original encoding does not change in mb_chars test.

See https://github.com/ruby/ruby/commit/9c1b3161f20162dc1682d758c0e4d2fc3a74745e

All Active Support tests pass in 2.0.0preview2 after this change.
Closes #8409

Override <%== to always behave as literal text rather than toggling based on whether escaping is enabled. Fixes that existing plaintext email templates using <%== unexpectedly flipped to *escaping* HTML when #8235 was merged.

Fix wrong doc and message about rails profiler.

Related to changes introduced in 553b5637

Updates to the Migration Guide

It's really comfy being able to copy-paste the file paths.

I suppose there's also some benefit in trying to figure out the file name as a small exercise, but I don't think that sort of thing belongs in "Getting Started".