Coding Standards: Use more meaningful variable names in Admin Ajax actions.

Per the [https://developer.wordpress.org/coding-standards/wordpress-coding-standards/php/#naming-conventions Naming Conventions]:

> Don’t abbreviate variable names unnecessarily; let the code be unambiguous and self-documenting.

This commit includes renaming of the following variables:
- `$post_ID` to `$post_id` — “Use lowercase letters in variable, action/filter, and function names”.
- `$ext_type` to `$extension_type`.
- `$ext` to `$extension`.
- `$id3data` to `$id3_data` — “Separate words via underscores”.
- `$msg` to `$message`.
- `$sb` to `$sidebar`.
- `$alt` to `$alternate`.
- `$pid` to `$post_id`.
- `$mid` to `$meta_id`.
- `$menu_obj` to `$menu_object`.
- `$noparents` to `$no_parents` — “Separate words via underscores”.
- `$sup` to `$supplemental`.
- `$cat_id` to `$category_id`.
- `$cat_name` to `$category_name`.
- `$out` to `$output`.
- `$r` to `$edit_result`.
- `$t` to `$taxonomy`.
- `$u` to `$update_result`.
- `$r` to `$result`.
- `$response` to `$response_data`.
- `$x` to `$response`.

Follow-up to [53723], [55365].

Props costdev, mukesh27, SergeyBiryukov.
See #64226.

git-svn-id: https://develop.svn.wordpress.org/trunk@61224 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov