Lock the compatibility mode in IE

[azaozz]

IE has several compatibility modes and several ways to control which mode is used. The admin is designed to support the "standards" mode for all IE versions.

We've been using a standards compliant <!DOCTYPE> to set that. However this may fail if there is any output before that tag, or if the user sets the mode manually with the button in the address bar in IE8.

In these cases some admin screens may not render correctly or even be quite unusable. To fix that we can add <meta http-equiv="X-UA-Compatible" content="IE=edge" /> that tells all IE browsers to run in the ​highest supported document mode of the browser.

[azaozz]

In 25575.patch: set X-UA-Compatible for IE. This doesn't make any difference in other browsers but shouldn't be set for Chrome Frame as the meta tag overwrites the header send from the server.

It also disables/removes the "Compatibility View" button in the address bar in IE8 and 9, prevents the users from accidentally switching to quirks mode.

[azaozz]

In 26029:

Add X-UA-Compatible meta tag in the admin, locking the compatibility mode in IE to the highest supported document mode. Fixes #25575.

[azaozz]

When there are conditional comments, the meta X-UA-Compatible tag doesn't work (bug in IE8). Best to set that as HTTP header as in 25575.3.patch. Alternatively can remove the conditional tags and add the ie8 class with some JS.

[azaozz]

In 26089:

Fix the X-UA-Compatible header for IE8, fixes #25575