Make dashboard more mobile friendly in 5.6.x

Re-adding viewport in whole dashboard, some tweaks in mobile.less
Make the asset helper easier to use

When using the asset library helper, it expects a file object for the existing file. Yet files are usually saved as file IDs. 

Making the helper adapt to accept an object or ID for the existing file saves anyone using the library from having to worry about the difference.
500 on validate_user_email.php tool

This file is no longer used anywhere in the core, and calling it (no matter what parameters) will cause an Internal Server error because UserInfo::validateUserEmailAddress() does not exist
make sure we only install new blocks
Don't show that a page is pending approval unless they can view the toolbar
Let's fallback to GD if ImageMagick fails
Move common Content functions to a new helper

Developers have to reuse the content of the ControllerBlockContent to correctly handle the HTML code.
Let's move this common code to a new content helper.
Fix setting editor width
Merge pull request #1696 from mlocati/add-content-helper

Move common Content functions to a new helper
Merge pull request #1685 from Remo/content-importer-block-types

content importer - only import new blocks
Merge pull request #1697 from mlocati/patch-1

Fix setting editor width
version bump
Merge branch 'master' of github.com:concrete5/concrete5
Merge pull request #1659 from JohntheFish/patch-6

Make the asset helper easier to use
Merge pull request #1684 from Mnkras/patch-4

500 on validate_user_email.php tool
Merge pull request #1688 from Mnkras/patch-6

Don't show that a page is pending approval unless they can view the toolbar
Merge pull request #1695 from mlocati/fix-imagemagick-handling

Let's fallback to GD if ImageMagick fails
Merge branch 'responsive-dashboard' of https://github.com/hissy/concrete5 into hissy-responsive-dashboard
Always enable localization cache

Localization cache is enabled only if the initial locale is not en_US.
But, mainly on multilingual sites, we may have an initial en_US locale,
and the final locale is set after the initialization of the Localization
library.

So, let's always enable the localization cache...
Faster autoloadCore checks

Loader::autoloadCore is called hundreds of times for each page: let's speed
it up by using case-sentivive strings comparision
(strpos is about 40% faster than stripos: see http://3v4l.org/3GC1s )