Migrate D6 i18n blocks

Problem/Motivation

Block translation is provided by i18n_block submodule of the i18n suite providing a language selector, a migration is needed to create language aware config entities.

Proposed resolution

We'll need a custom source plugin to handle the {i18n_blocks} table that identifies a block by "ibid", module and delta. It's probably a good idea to mirror what i18nblocks_db_rewrite_sql() does.
I18nstrings is used with the following keys:

blocks:block:$delta:title
blocks:block:$delta:body

We probably need to have a plugin source that gets all this information from both the {i18n_strings} and {i18n_blocks} tables.

Original report by Ryan Weal

i18n block (D6/D7 i18n contrib -> D8 block config). Provides a setting to specify that the block should only be displayed in a specific language OR if the block is plain text, to inject the block's data into the original language and expose that text in core's string translation facility.