Simplify Migrations/Table structure

[#594] Compress migrations - See https://github.com/browsermedia/browsercms/issues/594  (or release_notes.md) for upgrade instructions.

* All migrations from 3.0 to 3.5.4 have been compressed into a single migration.
* Remove previously deprecated create_versioned_table, remove_versioned_table methods
* 4.0 is a single migration as well
* Remove all upgrade scripts for previous versions.

[#639] Remove 'optional' table prefixing
* All core tables now start with cms_ which matches how mountable engines create tables (i.e. all model tables are namespaced to the engine) There was little value in ignoring rails conventions, which it generates a number of bugs and issues.
* Add migration for applying table prefix for core tables.
* Add deprecation warnings