Clean up of monthly thread summary pages

The monthly thread listing in Hyperkitty uses some styling from older mockups that had a gradient on them and generally I think doesn't match the work Karen Tang did on thread lists (for example, how the thread list displays on the mailing list overview page.) I put some work into updating the look of the monthly thread summary pages to better match other thread lists in Hyperkitty.

Here is the before:

Here is the after:

I went through the three sample mbox dumps I had available and clicked everywhere to make sure my changes to the thread list templates and CSS didn't affect any other screens in an adverse manner and I couldn't find any evidence of that.

Mobile testing + responsive layouts for mobile complete. Just did a rebase with master as of today, tested and working. This one is ready to go!