v5.6.0 Add support for migrations from LTI 1.1 to 1.3. Add deprecations in preparation of v6

What's Changed

• Abstract the code by @dbhynds in #101
• MGMT-132: Supporting migrations by @dbhynds in #99
• Initial preparation for v6.0 by @dbhynds in #105

NOTE: The recommended way for creating LTI Message Launches from a request has change. Instead of $ltiMessageLaunch->validate($request), instead do $ltiMessageLaunch->initialize($request). This will change permanently in v6.

Full Changelog: v5.5.4...v5.6.0