Missing image alt attributes in core

[afercia]

Searched for all the <img occurrences in core and found some places where images miss an alt attribute. In some cases they're a minor issue I guess. Though it's invalid HTML5, they don't seem to cause major accessibility issues. In other cases the lack of the required alt attribute is more important. For example, in Press This, when adding a suggested image in the editor in HTML mode there's a chance users will save the content "as is" and output in the front-end a bit of invalid, not accessible, code:

Worth noting as soon as users switch the editor to "Visual" mode, the alt attribute will be automatically added.

Other cases are really minor, for example the "locked post" dialog spinner:

Same for images used in the media templates, JavaScript templates, in TinyMCE, and so on.

It would be pretty easy to fix all the missing alt attributes, it's definitely not "rocket science", but I'm a bit unsure what to do here. Just one big patch to fix everything (have a patch already) or maybe specific tickets for each small bit of code that needs to be fixed? Any feedback more than welcome.

[afercia]

Forgot to mention two relevant cases:

• the About page
• the plugin contributors list in the Plugin detail modal dialog

[afercia]

Uploaded patch for the "fix everything" approach.

[afercia]

Refreshed patch. Adds an "Image preview" alt attribute to the image in the Image Editor (when resizing, cropping, etc.).

[joedolson]

This looks sound to me; I didn't verify the usage context of every single image applied here, but the ones where the usage was pretty obvious were definitely good.

[wonderboymusic]

In 35567:

Accessibility: add missing alt attributes to a gaggle of <img>s.

Props afercia.
Fixes #34583.