Update progress bar scripts and styles

* Correct label for "Import [flickr] content" checkbox
* Escape HTML in localizations as needed
* Update progress bar scripts and styles
Code format and tweakings

* Add setting options for Flickr User ID and Images Per Page
* Include [flickr-gallery] in Ajax importing
* Remove "Compatiblity" verbiage
* Remove most `debug_mode` checks
* Rename WP_AJAX call
* Update import examples
* Use `empty` vs. `isset` checks