invite -> invitation for content keys and for var names in tripel_web and tripel_core (still need to fix var names in old unit tests).  minor fixes to content key references in nodespace_edit.html.  implemented nodespace_invitation_create.html.  implemented user_invitation_create.html.  trpl-ng.js:  implemented user (metaspace) invitation creation, nodespace invitation creation, nodespace info editing, perm checking for available metaspace commands, some minor refactoring (renamed a couple states, updated expected REST response structure, etc).  util.py: implemented an async version of sendmail, but it doesn't seem to work yet.  tripel_web.py:  refactored some privilege editing page code into PrivilegesEditForm class, added json mode for a bunch of pages (which entailed some of its own refactoring in page classes).