User Settings import and export

[rushsteve1]

Support for exporting and importing user settings via JSON. Closes #1347

What this PR does NOT do:

• Bulk user migration
• Export sensitive data like passwords
• Create/manage user accounts
• Export feed data

This simply allows individual users to export and re-import their settings.

Have you followed these guidelines?

• [x] I have tested my changes
• [x] There are no breaking changes
• [x] I have thoroughly tested my changes and verified there are no regressions

I have tested by changes against my own install, but I know there are a lot of different setups so I could use some more help testing

• [x] My commit messages follow the Conventional Commits specification
• [x] I have read and understood the contribution guidelines

[fguillot]

Is it ready to be reviewed?

I'm not able to upload the JSON file.

Also, I don't see the need to duplicate the "Feeds Backup" section. The OPML import/export is already available in the "Feeds" menu.