[4.2] Smart Search: Add optimisations & garbage collection

[Hackwar]

Summary of Changes

There are situations where the index of Smart Search can get poluted with old data (unused terms and taxonomies, already deleted content items) and where the only solution so far has been to clear the reindex and start indexing all over again. This is not something that you really want to do in a live scenario with a large site, so this attempts some different solutions.
First of all, it runs a garbage collection, which by default deletes all entries, which aren't present in the base table of the base component. We then delete all orphaned term and taxonomy mappings as well as all then orphaned taxonomies and terms. In the end, we then execute the already previously present optimisation queries. To trigger all of this, a new dropdown button was added in the backend named "Maintenance", where you can choose between purging the index and doing the optimisation.
Note: The garbage collection has been added as a default to the Adapter base class and there could be situations where this results in a fail. I'm open for ideas here.

Testing Instructions

1. Apply the PR
2. Click on "Optimise" in the backend
3. Afterwards see, that searching still works properly.

Notes

1. Since 4.1 is in RC, this will have to go into 4.2. Since there is no 4.2-dev branch yet, I'm pointing this towards 4.1-dev for now.

[sakiss]

Would it be possible to add that task to the new com_scheduler ?
Removing that responsibility from the admin would be great.

[Hackwar]

This task is not something that you should execute regularly.

[nadjak77]

I have tested this item 🔴 unsuccessfully on 7a4097b

After applieing the patch I get an error in seachrindex - indes:
There is no "table.columns" asset of a "script" type in the registry.

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[Hackwar]

Can you please check if you need to do a composer install and/or npm i? This PR actually does not change anything in that regard, but maybe a previous change introduced a problem here?

[webnet-assmann]

I have tested this item ✅ successfully on 7a4097b

I've tested it with the prebuilt package and sample data. Searching was working before and after click on "Optimise".

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[joomdonation]

Tested and it worked OK. Just minor things with the code and I will mark my test result.

[joomdonation]

I have tested this item ✅ successfully on 49576c6

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[joomdonation]

The last change was a minor change, so I restored test result of @webnet-assmann. This PR is now RTC.

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[Hackwar]

I'm investigating why the api tests are failing. Please don't merge this right now. I want to check that first.

[Hackwar]

I found the issue and fixed it. This could be merged now.

[fancyFranci]

Thank you Hannes!