Closes #796: Auto generation of AVIF on save

[Miraeld]

Description

Fixes #796

Documentation

User documentation

Fix the issue related to the auto-optimization on save while AVIF is being enable.

Technical documentation

When we save the Imagify settings, we get to a callback which will check if the AVIF is enable or not.
If it is, we will start optimizing the images when AVIF is missing in order to generate these AVIF versions of images.

Type of change

• Bug fix (non-breaking change which fixes an issue).

Checklists

Feature validation

• I validated all the Acceptance Criteria. If possible, provide sreenshots or videos.
• I triggered all changed lines of code at least once without new errors/warnings/notices.

Code style

• I wrote self-explanatory code about what it does.
• I wrote comments to explain why it does it.
• I named variables and functions explicitely.
• I protected entry points against unexpected inputs.
• I did not introduce unecessary complexity.

Observability

• I handled errors when needed.

[remyperona]

There is some conflicts to fix after the merge of my PR to the main branch

[jeawhanlee]

@Miraeld Could you fix the codacy errors, they are mostly related to phpcs.

[Miraeld]

As noted by @Mai-Saad , if we run only this branch, we might get our file deleted.

However, merging this branch with this one fix the issue :)

So this PR requires #807 :)