Fix broken migration in MySQL

`keys` is a reserved name in MySQL, so if this migration actually attempted to remove any duplicate keys it would fail.

Closes #19344

See merge request !5005

import_url migration performance improvements

 Nullifying empty import_urls upfront so the number of projects with import_url not NULL decreases to 1/5.

 Also, now processing batches in blocks of 1000, with a threaded process - a bit experimental.

See merge request !5149

 Nullifying empty import_urls upfront so the number of projects with import_url not NULL decreases to 1/5.

 Also, now processing batches in blocks of 1000, with a threaded process - a bit experimental.

Added setting to set new users by default as external

## What does this MR do?
This implements the feature request #14508. It adds an option in the application settings to set new users by default as external.

## Are there points in the code the reviewer needs to double check?
Everything. Like I mentioned in the discussion of the issue my knowledge of Ruby basically doesn't exists. I tested it on my machine and it seems to work, but as I am very unexperienced in Ruby I highly recommend to  take a close look at the code.

## Why was this MR needed?
It was requested by @DouweM to work on the issue with the proposed changes by me.

## What are the relevant issue numbers?
This MR is for the issue #14508 that followed up after the implementation of #4009.

See merge request !4545

As requested by the issue #14508 this adds an option in the application
settings to set newly registered users by default as external. The
default setting is set to false to stay backward compatible.