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
Document that yaml files shouldnt be served directly
Include code to block yaml files in installer generated .htaccess
MINOR Formatting fix
Update docs/en/changelogs/3.0.0.md

minor typo
BUGFIX: for date manipulation use the SS_Datetime::now, otherwise it does not respect the mock date.
NEW: Allow configuration of send_all_emails_to, ccs_all_emails_to, and bcc_all_emails_to via the config system.
NEW: Added Email::send_all_emails_from() setting.
FIX: Set up the test mailer before loading the fixture, in case fixture-creation causes emails to be generated.
FIX: Fixed operation of inlined images in Mailer, when no inlined images actually attached.
API CHANGE: Added Security::set_login_url() so that you can define an alternative log-in page if you have made one yourself.
BUGFIX: Improved detection of empty HTMLText fields.
FIX: Removed notice-level error after forms w/ required fields are made readonly.
FIX: Added error message fields to default search form
MINOR: Fixed a glitch that causes warnings in cli-script execution.
BUGFIX: Prevent notice-level error in Session code when non-array is turned into an array.
BUGFIX: Better error message when 401 response is corrupted.
BUGFIX: Updated SilverStripe error handler so that log_errors still works.
MINOR: Minor fixes to FunctionalTest