Move focus items JS into main mtc.js library

Originally posted on github :
https://github.com/mautic/mautic/issues/2771

Currently in theme section, the delete option is available only for non default themes
Problem is that customers using mautic taking time to define multiple themes continue to see these default themes.

Would be cool if we could “hide” the default themes (I understand we can’t delete them maybe as the update process would recreate them at next update ?)

Link to Github seems correct (according to the title) but the body is wrong