Factor out helper code from prediction app

In order that we can use the same stuff in the new sea_levels app.

- Factor out `now_rounded` and `format_datetime` from predictions.views.helpers
  into api.libs.view_helpers
- Move SLUG_REGEX into django settings
- Move DATETIME_FORMAT into django.conf.settings
- Factor out URL query parameter parsers into new lib
- New parse_tide_level function
- Move exceptions out of predictions.views.exceptions

https://www.pivotaltracker.com/story/show/75795976 [#75795976]