Allow pre-setting snippet values from URL

This allows pre-setting snippet values from edit URL. Note that `attributes=` merges attributes (despite what `=` might suggest).

This makes integrating with snippets easier. Externally pre-filling existing snippets makes sense, because mirrored ones are created with no content by the CMS (https://github.com/comfy/comfortable-mexican-sofa/wiki/Using-cms-for-emails).