Exciting new features; improvements in migrations generation

• Fixed: on content creation, assigning a section by identifier instead of id
• New: allow setting section upon UserGroup creation and update
• New: when setting a value for a 'selection' content field, it is now possible to use the selection string value instead of its numerical index; it is also possible to use a single index/name instead of an array
• New: when using the generate command for content creation migrations, 2ndary locations are exported. Also, the sort field and sort order of the main location are exported
• New: it is now possible to use a specific type of exception during execution of a Migration Step (or a listener) to abort a migration without having it flagged as failed - it can be flagged as either done or skipped
• New: it is now possible to create/update/delete sections via migrations
• Improved many error messages