Migration service and script

[Elfangor93]

This PR adds the migration service as well as a new MVC routine called migration.
A migration script called Jg3ToJg4 to migrate a JoomGallery v3.x to v4.x is already included. Additional migration scripts intended to be installed as an extension like in Jg3.

The migration view can be requestid with the following url:
/administrator/index.php?option=com_joomgallery&view=migration

How to test this PR

Attention!!
This PR is not compatible with Joomla v4.4.0, v4.4.1, v5.0.0, v5.0.1.
Older and newer versions of Joomla are fully compatible.

Prerequirements

There has to be a source (JG3 updated to JG4) and a destination (JG4) available.

There are two possibilities:

1. Source and destination are within the same Joomla installation. For this you update a JG3 in a Joomla 3 to Joomla 4 and then update the JG3 to JG4 using this PR for the installation.
2. Source and destination are in two different Joomla installations, but on the same server. Here you prepare the source installation the same way as described in 1). Additionally you setup a second, fresh Joomla 4 installation. Within this Joomla 4 you install a fresh JG4. This second installation serves as your destination.

This way you end up having a source installation containing database tables looking something like #__joomgallery_XX_old. And a JoomGallery administration folder containing an xml file called joomgallery_old.xml. The source needs to have some categories and images.

Perform a migration

A migration consists of 4 steps.

1. Configuration
2. Pre-Check
3. Migration manager
4. Post-Check & Finish

You can go to the next step of the migration, if you successfully passed the prvious one.

Step 1: Configuration

Here you have to define where your source is located you want to use for the migration as well as how you want to write the records into the destination.

Source: JG3 updated to JG4
Destination: JG4

Step 2: Pre-Check

Your entire system and installation gets checked if everything is setted up correctly such that the migration will run without problems.

Step 3: Migration manager

Here you see a list of migration processes that have to be performed in a specific order. First the categories have to be migrated, then the images and at last the category thumbnails have to be reselected based on the new IDs created for the images during migration.

The start button starts the automatic execution of the migration process based on a queue. For each element in the queue an ajax request is started to execute the migration of this element. When the request returns from the server, the progress bar and the log output gets updated to inform you about the migration progress.
The automatic execution of the migration can be stopped at any time with the "Stop migration" button.

As soon as all the migration processes are performed the button on the bottom "Check and finish migration" becomes clickable.

Step 4: Post-Check & Finish

The results of the post check tells you how well the migration was done. It checks if all the queues are processed and if there were errors in the migration which might need to be resolved.
The button to remove the source data is not yet wokable. When you click it, you will always get a success message. But actually is not doing anything currently.

[AlexanderSupp]

I have prepared a test environment for this pull request.
My steps are documented here: http://joomgallery4.fotofreunde-much.de/de/administration-de/migration-de
I hope this works well. I'm looking forward. Likewise, I look forward to your progress.

[Elfangor93]

Yes, this is known. Im working on it.

[Elfangor93]

Thank you everybody for helping!