Simplify back office `rewrite` rule.

`rewrite` takes a regex to match against the URI, but since we don’t
care about what the URI is we just use a `^` which means “beginning of
string” and thus matches everything.

Simplify installation sandbox `rewrite` rule by removing the second
capture group, since it's not being used.

Improve product images `rewrite` rules.

Consider the following example for easier reading:

```regexp
^/([0-9])([0-9])(-[_a-zA-Z0-9-]*)?(-[0-9]+)?/.+.jpg$ /img/p/$1/$2/$1$2$3$4.jpg last;
                       ^/(\d)(\d)(-[\w-]+)?/.+\.jpg$ /img/p/$1/$2/$1$2$3.jpg last;
```

* Replace `[0-9]` with `\d`.
* Replace `[_a-zA-Z0-9-]` with `[\w-]`.
* Remove the last capture group `(-[0-9]+)?` as it is already matched
in the previous capture group.
* Properly escape the dot character (`\.`) used in the file
extension: `\.jpg`.

Improve category images `rewrite` rules.

* Unlike product images, we only need to capture what is in the second
level of the URI, so there is no need to have multiple regular
expressions to achieve this task.

Taking into account previous rules, the list of allowed characters is
set to include any alphanumeric characters `\w`, the dot `.`, and the
hyphen `-`, simplifying the whole capture group to: `([\w.-]+)`

* Properly escape the dot character (`\.`) used in the file
extension: `\.jpg`.