Layout/rendering separation

[benwarfield-usds]

Some template and CSS changes to separate the rules involved in the pseudo-frame layout from those involved in the rendering of individual messages.

• added a wrapper div to take most of the layout rules off of body
• changed selectors for rules inside the messages div to make them depend on message-container instead
• added rules to italicize channel-join and channel-topic messages
• reformatted the template to correct the indentation problem introduced by the wrapper-div commit (intentionally separated to make it easier to read).

Note: the most interesting changes are all bundled together in b70fc5c6b57ebd731f0d495b5eebaf7cb60ec480, which is probably easier to read (especially with respect to the changes to viewer.html) than the overall diff.

[hfaran]

Whoops, thought I had already approved this earlier. Will do so now.

I'll also publish a new release so your recent changes are available through PyPI.