Use default css file name.

Change the css file used from playdate.css to application.css, since
Rails compiles the latter by default, but not the former.
Allow HAML templates.
Use HAML for the players templates.
Use HAML for the availabilities templates.
Convert remaining ERB templates.

For the bulk of this conversion, erb2haml was used. After that, some
touch-ups were made to fix syntax errors and awkward constructs.
Make feed_table a non-partial.