Store previous image size before WebP conversion

[mitogh]

As part of the set of measures we want to measure and use to comparison to know the real benefits of installing the plugin and enabling the modules, we need some sort of metrics as discussed on:

• Define a list of metrics that we need to measure #63

For new and existing images as described in:

• Create WebP images on upload #22
• Regenerate existing images #24

Storing the size of the image before the conversion would be helpful in order to run diff in size per image against the current image size and also an overall total in savings by using WebP images.

This can be done in all available image sizes as well in order to provide a better metric for the whole site instead. The ticket itself references the previous image size but it would be more accurate to store all previous image sizes available on the site before the conversion.

[mitogh]

Slightly related to:

• https://core.trac.wordpress.org/ticket/49412