Enable Markdown deck description by default

The option to interpret deck description text as Markdown was added more than 5 years ago in https://github.com/ankitects/anki/commit/9ce4b219352136e2829e2400cca6732ed8d159b6.

The plan was to enable it by default eventually. I think the time has come for that.

Related: #5481

Read commit 9ce4b219352136e2829e2400cca6732ed8d159b6 and related issue #5481 to locate the existing deck-description Markdown option and its default setting. Done means the option is enabled by default and deck descriptions are interpreted as Markdown without requiring a setting change.

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