major restructuring of files; see notes

- deleted unused app.css and bootstrap css

- moved contents of ui directory to scss/exai to discourage overwriting exai files. new development should occur in the now-empty ui directory

- set exai defaults

- create _exai.scss, set up similarly to _inuit.scss with grouped imports and TOC
fix sidebar display

was set to display if exai_display_sidebar() returned FALSE instead of true (for testing sticky footer)