Don't show alert in alert bar on save

Instead of showing an alert in the alert bar when the story is saved,
just flash the last saved message in the upper-right-hand corner.

Also, flash the last saved message when assets are saved.

Addresses #548
Break alert colors into variables
Green background and text for last saved message

Addresses #548
Merge branch 'develop' into 548-more_subtle_save_alerts
Toggle save button style on save

Instead of flashing a last saved timestamp, change the button color to
green and the text to "Saved".

Addresses #548
Refine save button behavior

Toggle the ``disabled`` attribute on the button when it is displaying the
"Saved" message.

Use a modified version of the
[JavaScript Pretty Date] (http://ejohn.org/blog/javascript-pretty-date/)
function to format the timestamp in human readable form.

Use ``setInterval`` to continually update the timestamp.

Remove methods and code which wrote the timestamp to a separate element.

Addresses #548
Use icons to indicate saving state

As suggested by @patternleaf, instead of turning the button green to
indicate a successful save, use a check mark icon.  This avoids the issue
raised by @jwirfs-brock about the green color compelling the user to click
the button when it's in the "saved" state.

This commit also introduces a "saving" state which the button goes into
when clicked to give the user some kind of instant feedback. The button
does not go into the "saving" state during an autosave because there isn't
presently any signal sent for all of the different content types to
indicate that a save is being initiated.

Addresses #548
Position last saved tooltip to the right

Position the last saved tooltip to the right of the save button instead
of above it, per @bpawlak's suggestion.

The position, as well as other options for the tooltipster jQuery plugin
can be passed via the ``tooltipOptions`` property of the options of
``LastSavedView``.

Addresses #548
Set min-width on save button

Set a min-width on the save button so it always takes up the same amount
of space regardless of state.  This helps alleviate some of the overlapping
tooltip behavior as the state changes.

Addresses #548
Merge branch '548-more_subtle_save_alerts' into develop