Improve pagination.

Use django-pagination on both search results and speaker pages.
added keyboard navigation instructions to individual speeches
extracted heading class to mixins file
added keyboard navigation instructions to sections template, made css respsonsible more suitable for reuse
made sidebar section nav buttons consistent, added hover effect to keyboard navigation instructions
refactored to allow for remote hover effect
added cleared utility class
replaced old buttons markup with new consistent markup
refactored buttons for navigating speeches and speech sections
added new disabled button styles
styled pagination
Merge remote-tracking branch 'origin/issues/168-pagination'
Merge remote-tracking branch 'origin/issues/172-consistent-button-styles'