Migrate wiki pages on UI reference

Created by: seancolsen

This PR migrates the following wiki pages into one MkDocs page:

• https://wiki.civicrm.org/confluence/display/CRMDOC/UI+Elements+Reference
• https://wiki.civicrm.org/confluence/display/CRMDOC/Accordions
• https://wiki.civicrm.org/confluence/display/CRMDOC/Buttons
• https://wiki.civicrm.org/confluence/display/CRMDOC/crmDatepicker
• https://wiki.civicrm.org/confluence/display/CRMDOC/Icons
• https://wiki.civicrm.org/confluence/display/CRMDOC/In-Place+Field+Editing
• https://wiki.civicrm.org/confluence/display/CRMDOC/Notifications+and+Confirmations
• https://wiki.civicrm.org/confluence/display/CRMDOC/Section+elements

No new content or significant changes. Just import and clean.

Reviews welcome. I'll merge this on 2017-08-15 if there are no comments here.