This adds a setting in the My Account page for controlling whether

Markdown is enabled by default. When enabled, all text fields will

default to Markdown mode. When disabled, only fields already set for

rich text in the database will be in Markdown mode by default.
The default for users will be based on the 'default_use_rich_text'

setting in siteconfig. Right now, this is not exposed in the UI, but it

will be in a separate change.
There were a few places left in the codebase where we were assigning

default values for rich_text fields, and those have been removed,

allowing the UI to control the default based on this setting instead.

Saw that the default for the setting was to enable Markdown.
Edited text fields set for rich text and plain text in the database, and

both were set to edit in Markdown by default.
Disabled the default. Saw that that choice persisted.
Edited text fields set for rich text and plain text in the database. Only

the ones set for rich text were set to edit in Markdown by default.