Change default timezone to UTC

Commit 107ce3e3721c5517f345595a5740f2108e40e015 changed the pytz requirement to
be >= 2012h because versions earlier than that do not support the US/Pacific
designation of the timezone, which Review Board was using as a default. This
makes it difficult or impossible to install Review Board on Linux distributions
with an older version of pytz.

This patch changes the default timezone to UTC, which should be available in
all versions of pytz since time out of memory. With this, we can drop the pytz
strict version requirement as well, trusting that the platform version will
support UTC.
Merge branch 'release-1.7.x'
Lighten the deleted lines color in the diff viewer.

The deleted lines color in the diff viewer was still a lot darker than
the insert and replace lines, and the contrast with the move flags
weren't great. This lightens it a bit more, making it appear a lot
closer to the other colors, and making it easier to read.
Improve the style of the move indicator flags.

The move indicator flags were very noisy things. There was a flag for
every moved line, which made it harder to see that a range was moved as
a whole, if two ranges were right up against each other.

This changes the design to only have a flag for the first line in a
range, and not subsequent lines. Those are indicated by a tail on the
flag, or a flag pole perhaps.

This reduces the visual noise considerably, at the expense of only
allowing the first row to be clickable to jump lines. That's a necessity
due to the limitations in how we can represent this style in CSS (as we
cannot properly represent each row of that flag pole as an element with
the proper height in a TD without javascript).