Updates after #1806

After #1806, the `endAction` event callback introduced in #1795 was
made irrevelant. I introduced the callback to only submit the form
after the tip value was validated, but since we're now using a
`<form>`, HTML5 browsers already validate before submission.

Since we're now using `form.submit()`, I updated the cancel link to use
the native `form.reset()` behavior.

Again, since we're now using a `<form>` I figure we can use the
already-provided `input.defaultValue` instead of our hand-made solution
to track the previous value.

While making these changes I noticed that the cancel link has the
standard `<a>` mouse pointer even when it's disabled, so I updated the
CSS to fix this.

This commit also fixes a bug introduced in #1806 by not updating
`my-tip-bulk` to use `<form>`.
Merge pull request #1810 from gittip/updates-after-1806

Updates after #1806