Allow to exclude templates in webspaces

[danrot]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Fixed tickets	---
Related issues/PRs	---
License	MIT
Documentation PR	sulu/sulu-docs#462

What's in this PR?

This PR allows to exclude certain templates in webspaces.

Why?

Because some templates might not be allowed in all webspaces.

Example Usage

<?xml version="1.0" encoding="utf-8"?>                                                                                                                                                                                                                                          
<webspace xmlns="http://schemas.sulu.io/webspace/webspace"                                                                                                                                                                                                                      
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"                                                                                                                                                                                                                 
          xsi:schemaLocation="http://schemas.sulu.io/webspace/webspace http://schemas.sulu.io/webspace/webspace-1.1.xsd">
    <!-- ... -->
    <excluded-templates>                                            
        <excluded-template>default</excluded-template>
    </excluded-templates>
    <!-- ... -->
</webspace>                                

To Do

• Create a documentation PR