Add Edit All link to Differential revision

Summary:
Some text editors support opening multiple files at once.
I've used space as paths separator which may be compatible with some other
editors (I didn't tried any other though).
Note: This approach is incompatible with spaces in paths.
I am fine with changing it to anything else to support such paths or more
editors.
Probably the cleanest solution (yet still incompatible with most editors) would
be to use something like editor://open/?file=A&line=1&file=B&line=2 but it
would require also changing the way how it's configured and I think it's not
worth it.
BTW, I've used a hacky bookmarklet for this feature before.

Deleted or added paths may not exist in users filesystem but we don't know which
so the button tries to open everything.

Test Plan:
Click Edit All.
Delete Editor Link in settings, verify that the button is missing.
View diff without revision, verify that the button is missing.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1741