Change default config to use journal key

[micahellison]

Feature Request

Use Case/Motivation

When a user runs jrnl for the first time, it creates a config file with a default journal based on the user's input. That shows up under the journals key as default: (path) like so:

journals:
  default: C:\Users\micah\journal.txt

This is fine, except it can be a bit confusing for users that want to add per-journal configuration elements, e.g. different templates for different journals as seen in #1152. It would be easier to modify if the journal key was used instead, like so:

journals:
  default:
    journal: C:\Users\micah\journal.txt

[stale]

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