deposit: refactoring fixes

* Fixes unneeded base of base template introduced in 461039d.

* Fixes several JavaScript issues introduced during refactoring of assets as well as dynamic field lists.

* Fixes tag template issue in field lists.

* Fixes issue when appending new elements to field lists.

* Fixes typo in message variable names.

* Fixes uninitialized variables due to strict mode.

* Removes Invenio CKEditor configuration as this is configured by each textarea field in the data-ckeditor-config attribute.

* Adds SPAN-tag to whitelist of HTML-tags to support MathJax integration in CKEditor.

* NOTE Changes deposit/form.js module to export multiple methods. If your instance templates invokes ``form(configuration)`` it must be changed to ``form.init(configuration)``.