Add single file import export

[EgleKat]

What does it do?

Added two buttons to the diff viewer to import and export the config for individual files and content types

Why is it needed?

I find that usually I only need to apply changes to 1 content type.
I always check the diff viewer to confirm the changes that need applying, after which I need to close the diff viewer, un-tick all the content types in the table and find the required content type again, tick it's row, scroll all the way up and then click import/export. That is a lot of clicks!

How to test it?

1. Click on a single file row in the plugin table
2. Click on Import. Observe the file gets imported into the DB and the table updates
3. Click on Export. Observe the file gets exported from the DB into the config-sync directory and the table updates

Related issue(s)/PR(s)

Let us know if this is related to any issue/pull request