Configurable ISO 8601 date format string

I don't like M/D/YY so wanted to change the format string. I figured the best
way to do this is to make it configurable. Also, I changed the default to be
ISO 8601 since I think this is what most coders will be used to.

Test plan:

 * Opened blame and saw it's now in ISO format
 * Closed blame
 * Went to config and changed it back to M/D/YY
 * Opened blame again and saw it reverted to old confusing format ;)
Resizable panel with reasonable default width

Some of my collaborators have long last names which makes a dynamic width
annoying. Because of this (and another reason, see below) I decided to
implement a static-width panel that's resizable.

This commit adds a resize-handle and some style tweaks to make things look good
at varying widths.

The other reason a static-width panel is better is because when blaming large
files in repositories with lots of history there is a noticeable delay before
the panel appears. It would feel much better if it displayed instantly, but
this is problematic in the current implementation because the width is unknown
until blame returns.

With a static panel width, the panel can react immediately to the keypress and
display a throbber to indicate blame is running. Then when blame completes, the
panel can update without resizing.

I haven't actually implemented instant display yet but I intend to.

Test Plan:

 * Opened blame, saw long names had ellipsis
 * Resized panel with mouse
 * Scrolled down and up, resize-handle still usable
 * Edited some lines and made sure 'no commit' lines look OK
Factor out .stretch() to mixins.less
Show first 7 characters of hash

Since the actual value of the hash isn't useful after some amount of bits, I
looked up how many characters GitHub uses in their UI. It's 7. If that's good
enough for them I think it's good enough for us too.

Test Plan:
 * Edited a line
 * Reloaded window,
 * Saw only 7 characters in hash
 * Saw only 7 hyphens in my edited line
Merge pull request #13 from dmnd/static-width

Resizable but static width panel