Add mutable false to default refund reason record

[mdesantis]

Fixes #2573

[kennyadsl]

In another PR we could remove this mutable thing, if it's just used to do not allow admin to change its name (since it has to be the same of the constant defined here) in favor of a default boolean on the refund reason table. WDYT?

[mdesantis]

If the mutablething is used just to mark the default record yes, it shouldn't be much more complex than rename mutable to default and swap the values; if otherwise the mutable thing is there to represent some separated logic we could add a default column in order to manage this record and leave the mutable thing alone.

[jhawthorn]

Thank you!