First cut of LtiResource init now working

Have domain-level launch from coursesites now working to demonstrate
the process so far.
Added LTi instructor authentication store + filter

The LTI launch process now stores the identity of the user in the
HTTP session, keyed on the LtiResource being accessed so that a
user can have multiple identities simultaneously. This filter
retrieves the authn data from the session when accessing a
particular resource.
Removed original LTI auth filters
Tidied up recent commits a bit.

Includes cleaner separation of LINK & DOMAIN LTI launches,
and a new enumeration for these 2 types of launches.

The existing LINK launch URL has been simplified to remove
redundancy; the existing URL will be kept for backward
compatibility.