Convert body to use text_long instead of string_long

Problem/Motivation

The body field on the base entity table has a serialised array of value & format stored in a string long field. It has been pointed out that a text_long would be the appropriate type for this (credit @Berdir - thank you!). Without this change it can cause issues for those using the rest API for instance.

Proposed resolution

Create new base storage definition and migrate content from the old to the new then discard the old.

Remaining tasks

Test.

User interface changes

Minor label tweaks.

API changes

Anyone using the 'body' value directly modifying the module will need to use 'message' instead. No methods or functions are changing.

Data model changes

New field 'message' to replace 'body.