[BUGFIX] Use websiteTitle of site configuration for snippet preview

[RinyVT]

Summary

This PR can be summarized in the following changelog entry:

• Defining the website title within the snippet preview previously was only taken from websiteTitle of a site's language, now also the global websiteTitle is taken into account

Relevant technical choices:

• Site attribute is taken from TYPO3_REQUEST and the websiteTitle is retrieved from getConfiguration just like in core

Test instructions

This PR can be tested by following these steps:

• Pull branch
• Set websiteTitle within the site configuration
• Check a (sub) page of the site and start typing in the seo_title field, the website title should now stay in place

Quality assurance

• I have tested this code to the best of my abilities
• I have added unittests to verify the code works as intended

Fixes #549