Fix another SVN unicode problem.

When there's a file-not-found error with unicode paths in SVN, we were trying
to format the encoded string into a unicode string to create the error message.
This caused a UnicodeDecodeError when it tried to coerce it using the 'ascii'
codec. I've changed the `FileNotFoundError.__init__` method to cope with paths
which are bytes as well as paths which are unicode.

Testing done: Ran unit tests.

Fixes bug 3425.

Reviewed at https://reviews.reviewboard.org/r/6326/
Fix some Python 3.x issues that have slipped in.

Since I initially did our 3.x porting, a few things have slipped in (mostly
merges from the 1.7.x branch, which doesn't have the same requirements).

Testing done:
Ran 2to3 and saw that the only reported issues are expected (presence of
__future__ stuff, and code where we use six to update something but 2to3 can't
tell the difference).

Reviewed at https://reviews.reviewboard.org/r/6327/
Issue 2766: Web api 'review-requests' not returning draft review requests

Added check for whether the user is admin in webapi/review_request.py. If so,
the _query function for ReviewRequestManager (reviews/managers.py) will query
for both public and NON public (draft) review requests, as opposed to just
querying for public ones.

Testing Done:
- Verified that admin accounts see draft reviews through
  /api/review-requests/?status=all, but normal accounts do not.
- This change only affects the webapi, and not any other module that might use
  the ReviewRequestManager. Verifiedthat the "All Review Requests" page is
  unaffected here.
- webapi and reviewboard module unit tests run and passed
- Added unit tests for private review request visibility in the reviewboard
  and the webapi unit tests

Bugs closed: 2766

Reviewed at https://reviews.reviewboard.org/r/6328/
Issue 3532: Linking to a collapsed review does not expand the review

Linking to a collapsed review does not expand the review. Fixed by parsing the
url and force the linked box to open.

Testing Done:
- Tested on local server, add /#review{review_id} to url will trigger the
  corresponding box to open.
- All unit tests passed.

Bugs closed: 3532

Reviewed at https://reviews.reviewboard.org/r/6332/
Add a confirmation when discarding a pending review from the review dialog.

When a user has a draft review, and opens up the "Edit Review" box to finalize
their comments and publish, the "Discard Review" command did not confirm at
all. Now it gives a prompt to discard or cancel.

Testing Done:
Tested visually, by publishing a review; both pressing and not pressing 'ok'
under the comment.

Case 1: Clicking Discard Review, then clicking cancel. It takes away the prompt.
Case 2: Clicking Discard Review, then clicking discard. Discards the review and
goes back to the review request.

Bugs closed: 3300

Reviewed at https://reviews.reviewboard.org/r/6341/
Fix discarding of comments in the diff viewer when double-clicking flags.

When double-clicking on a line number to add a comment in the diff view, this
is registered as two seperate clicks. The comment dialog opens on the first
click, and then on the second click the dialog is closed, the comment is
deleted, and then the dialog is reopened (but no new comment is created). So,
clicking save would not save the comment.

To solve this:
Before the comment dialog is shown, first check to see if there is already a
comment dialog active. If there is, and it is a comment dialog for the same
block as the one that is about to be shown, then don't show a new comment
dialog (keep the old one as it displays the same comment). A double-click is
registered as two clicks for the same comment block, and so the first click
will show the dialog, and the second will do nothing.

Testing Done:

Tried all of the following (in Firefox, Chrome):
 - Adding comment by double-clicking on a line number and comment flag
 - Adding comment by single-clicking on a line number and comment flag
 - Adding comment by selecting a range of lines

All JavaScript unit tests passed.

Bugs closed: 3437
Reviewed at https://reviews.reviewboard.org/r/6340/