Improve UX for bulk actions on translations page

[nacin]

The UX for bulk actions on the translations page is not ideal. To (de)select all, you must click "Bulk" then a button. This causes the mouse to move farther than necessary, and is an extra click. It is also not a typical user experience here. Typically, the table heading row has a checkbox which selects and de-selects. Notably, of course, this is how WordPress? core does it.

Placing the checkbox in the header will also make it a bit more obvious (in the UX and IA sense) that it is only being applied to the current page, which renders the tiny-font note useless.

Additionally, you should be able to shift-click multiple checkboxes in a row.

Finally, "Approve Selected", "Reject Selected," and "Translate via Google" should all be merged into a bulk actions dropdown with an Apply button, again following core. This provides for fewer clicks, which is the whole point of bulk actions to begin with, not to mention a more consistent user experience when compared to WordPress?.

(In [680]) Introduce a checkbox in the table header on the translations page to control select all/deselect all.

Also provide for shift-clicking multiple checkboxes in a row.

see #188.

188.diff​ moves bulk actions into a dropdown. By breaking it into a separate form, it completes a TODO and removes quite a bit of JS, some of it pretty hacky.

It also hides the checkbox column when ! $can_approve.

Before:

After:

Big +1 from me. Goes in the direction of ​http://blog.glotpress.org/2011/10/18/rethinking-the-glotpress-ui/ which is a good thing to keep in mind for future UI tweaks.

(In [681]) Move bulk actions to a dropdown. fixes #188.