add modal dialog & dynamic form to Batch Actions

thanks to @chrise86 for the updated CSS so this isn't horribly ugly
add docs

+ rename from `modalDialog` to `modal_dialog`
+ use the default confirm message on the form if none given
update batch action translations
Merge pull request #1815 from seanlinsley/1815-batch-action-forms

Batch Action forms
small docs update
replaced `eval` with alternatives using `define_method`
replace *_eval with *_exec