fixes #33 - set a term(s) status via the keyboard

[robby1066]

This PR makes the following changes to /static/js/lute.js:

Adds a new function called increment_status_for_marked_elements

This function collects selected term elements, groups them by their data_status_class level, then submits them in batches to be updated with update_status_for_elements.

This function accepts a single argument shiftBy, an integer indicating the amount and direction to increment the status class as defined in the constant validStatuses.

Abstracts the code in update_status_for_marked_elements into a new generic function update_status_for_elements

Separating the selection of elements from the code that submits them to the server allows other functions to update statuses for different use cases.

update_status_for_elements takes two arguments

• new_status: the status that will be set on the elements
• elements: a nodeList, array, or jQuery selection of elements to update

Passes a new argument to update_selected_statuses

update_selected_statuses was doing a redundant selection of elements to update what was sent the server by update_status_for_marked_elements. Passing an elements argument instead allows more flexibility upstream and removes duplicated element selection code.

Binds arrow up and arrow down keys to increment_status_for_marked_elements in handle_keydown

[jzohrab]

Looks great, thanks. I'll add a test in the dev branch.

[jzohrab]

Had to make one small change: setting the status to 0 isn't allowed, as that implies a missing term. The model isn't clear on this point, my spec was bad.