Update sitemap module sitemap.php

This fix is included to solve this bug: https://github.com/pyrocms/pyrocms/issues/2838
Now the function check if there are posts with status "Live", if the count is 0 then the loop doesn't print the blog link to the sitemap
Added the Field field type. Allows to use other fields while submitting entries.
Added the Template field type. Allows to display data in the admin from other columns using Lex tags.
Added the an argument to disable the field_assignment_construct if necessary. Needed for the Variables migration.
Added Convert variables to streams migration
Updated the Variables module to use Streams
No need for views. This is Streams based.
I love Eloquent but this model is no longer needed :-)
Fix - Use query builder to insert folder and get the id
Fix - Schema builder sets prefixes properly.
Fixed some streams_m bugs with schema builder. BOO dbforge sucks.
Fix - check if entry as been updated if strtotime > 0
Illuminate\Database\Schema\Builder depends on Doctrine\DBAL\Schema\MySqlSchemaManager but it was not specified as a require in the Illuminate composer.json. So we need to add it here.
Fix “unknown database type enum requested”

	* Should probably be addressed in Doctrine, if not in Illuminate\Capsule
	* Doctrine 2: Resolving “unknown database type enum requested” - http://wildlyinaccurate.com/doctrine-2-resolving-unknown-database-type-enum-requested
	* Unknown enum database type in migration file / Doctrine issue - https://github.com/laravel/framework/issues/1346
	* Error when mapping database enum field type - https://github.com/symfony/symfony/issues/866
Oops forgot to get the class name.
Update index.php in install folder

I have added a control version for php.
Users should kwon which php version is required to install PyroCMS before run composer install in order to avoid the last step problem.
If users try to install Pyro with a lower version the installation process stops to the 4th step without any message.
With this fix users already kwon wat they need
Merge pull request #2898 from ChristianGiupponi/patch-3

Sounds like a good idea to me.
Merge pull request #2885 from obrignoni/2.3/feature-variables-to-streams

Let's get this bad boy in.
Add "Add" button for variables
Send to edit/-id- after creating and protect against cancel_uri false id