Editor: Fix bug adding tags on Hebrew keyboards

[nylen]

Fixes #740. The Hebrew letter "ת" is in the position of the comma key on US keyboards. As a result, pressing this letter in the editor tags control will add the currently typed tag instead of adding the letter.

Handling special keys like arrows and Enter in onKeyDown based on their keyCode works well, but for comma and other keys that result in typed characters, we should use onKeyPress and charCode to correctly determine the character that was typed.

This PR also fixes a bonus related bug: currently it is possible to add tags consisting of only whitespace by pressing the comma key.

To test

Switch your keyboard layout to Hebrew. For those unfamiliar with this process, like I was, here's a poor-quality GIF demonstrating how to do it on OS X:

Navigate to the post editor and activate the tags control. Type a few letters and then press the comma key to send "ת". Verify that the letter is typed instead of adding the currently typed tag.

Also, type a tag consisting of only one or more spaces, then press the comma key and verify that this does nothing instead of adding an empty tag.

[aduth]

Confirmed that this works well, including existing comma behavior for both U.S. and Hebrew keyboards (where comma is in place of ' on U.S. keyboard).

Found this to be a pretty handy tool for testing varied behavior of keyboard events: http://www.asquare.net/javascript/tests/KeyCode.html

One question: Should we expect similar conflicts with other keys with specialized behavior, such as tab or arrow keys? If so, should we move most of our handling out of onKeyDown and into onKeyPress?

If the above is not a concern, I think this is good to merge 👍

[nylen]

The keys you mentioned (tab and arrows) do not generate keypress events according to your test page or this similar one: http://unixpapa.com/js/testkey.html

A good rule of thumb seems to be this: for typed characters (that keyboard layouts could rearrange), use keypress; for special keys (that keyboard layouts do not rearrange, as far as I know), use keydown.