BUG: Right click on LeftAndMain menu caused CMS preview window to open

The onclick event for LeftAndMain menu links didn't check if the click
was left or right, meaning that right click events could trigger the
function for loading split view mode in some browser/os combinations.
NEW Preview size and default mode configurability
Clarified docs on isDev usage in dev/build
FIX PHPUnit latest not working with composer installed builds

When using composer, we must rely on the composer autoloader to
load in PHPUnit and not try do to so ourselves, as the old
PHPUnit\Autoload.php file doesnt understand how to find things
in vendor
Merge pull request #1117 from silverstripe-rebelalliance/fix/phpunit/3.0

FIX PHPUnit latest not working with composer installed builds
API Remove Content-Length setting from HTTPResponse (fixes #8010)

This reverts commit 356a367eb5d05bea3dfa2edaeabc52a2496b93b2.
We can't use headers_sent() to determine an accurate
content length, since PHP defaults to buffering a couple of bytes
even without ob_start() (see "output_buffering" setting).
This makes the patch harmful, since it breaks any responses relying
on more structure data, like removing closing brackets from JSON.
Which in turn breaks the CMS in horrible ways (see #8010).
See #7574 for context.
Fix typo in templates/Controller.ss
Corrects indentation and line length
BUG Exception handling and email notification mechanism now correctly considers the stacktrace as provided by the exceptionHandler function, instead of attempting to perform a debug_backtrace further down the reporting chain (which ends up generating an unnecessarily nested stacktrace). Debug was cleaned up so that errorHandler and exceptionHandler both act consistently. As a result, the LogErrorEmailFormatter class could be simplified.

This was required to fix a bug in which exceptions would not have a visible stacktrace when handled by the email logger.
Added test cases for exception error logging
API HTTP::urlRewriter with (string)$code deprecated in 3.1. Fixed regressions and CSS urls.

urlRewriter will expect a callable as a second parameter,
but will work with the current api and simply raise a deprecation error.

HTTP::absoluteURLs now correctly rewrites urls into absolute urls. Resolves introduced in c56a80d6ce

HTTP::absoluteURLs now handles additional cases where urls were not translated.

Test cases for HTTP::absoluteURLs added for both css and attribute links.

Cleaned up replacement expression and improved documentation.
BUG Respect CMS preview active state (related to #8089)
API: Use HTMLText as default for RSS Description

To allow for custom RSS descriptions (i.e. provided by a method rather than a DB field) and avoid double escaping a better default class for Description is 'HTMLText'. http://www.silverstripe.org/customising-the-cms/show/11606#post319519
Document that yaml files shouldnt be served directly
Include code to block yaml files in installer generated .htaccess
MINOR Formatting fix
Merge pull request #1141 from drzax/patch-1

MINOR Formatting fix
Merge pull request #1140 from silverstripe-rebelalliance/yamlaccess_30

Document that yaml files shouldnt be served directly
Update docs/en/changelogs/3.0.0.md

minor typo
Merge pull request #1101 from chillu/pulls/preview-configuration

NEW Preview size and default mode configurability