Followup: Enable/disable additional diagram formats using Kroki

What does this MR do?

Allow to enable/disable additional diagram formats that require a companion container.
This merge request is a follow-up of !44851 (merged)

Resolves #241744 (closed)

Screenshots

Added a new section "Kroki additional formats" to enable or disable additional diagram formats:

In the screenshot below, BlockDiag is disabled:

And now, BlockDiag is enabled:

Database

Migration #up

== 20201120092000 AddKrokiFormatsToApplicationSettingsTable: migrating ========
-- change_table(:application_settings)
   -> 0.0040s
== 20201120092000 AddKrokiFormatsToApplicationSettingsTable: migrated (0.0040s) 

Migration #down

== 20201120092000 AddKrokiFormatsToApplicationSettingsTable: reverting ========
-- remove_column(:application_settings, :kroki_formats, :jsonb, {:null=>false, :default=>{}})
   -> 0.0044s
== 20201120092000 AddKrokiFormatsToApplicationSettingsTable: reverted (0.0063s) 

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done