[C2] Fixes to settings migration

[ywarnier]

These are settings.variable that are different in a system freshly installed vs a system migrated from 1.11 to 2.0:

• changeable_options
• show_tabs
• tool_visible_by_default_at_creation
• active_tools_on_create
• visible_options
• showonline
  (practically all the settings with type = checkbox)

In a new installation, these fields have a selected_value as a comma-separated list of strings.
In a system migrated from 1.11.x, these appear as multiple records, except for show_tabs which appears as ['some value', 'other value'].
It should be as in a fresh C2 install: separated by comma, only one record per URL in the settings table.

On a newly installed C2, "type" is empty for all settings, but this is normal as the type is now defined in the Schemas in src/CoreBundle/Settings/*SettingsSchema.php.

Add a new migration to fix those in C2 (change from multiple to single record and fix show_tabs).

[christianbeeznest]

It is checked with this commit 84fee6b

Thanks for confirmation.