Add and restore context in recorder [MIGRATION]

[balloob]

Description:

This stores context in the database and restores it when being retrieved.

This requires a migration and might take long. I added a new migration step "add columns". For MySql it will try to add all columns at once, for SQLite it will need to do 1 at a time.

CC @OverloadUT

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass

If the code does not interact with devices:

• Tests have been added to verify that the new code works.

[randellhodges]

The add columns method fails against MSSQL. The word "COLUMN" breaks it. Probably needs a workaround like the drop_index method uses.

See here for the post I created over on the home assistant community.

[dashony]

hi guys, not sure if it is to do with this add-on but since I've upgraded to v0.76 I'm getting "invalid config" ,. I have not made any changes to these 3 features. (using HASS.io on Pi2)