chore(*): fix serving of URI-encoded files on code.angularjs.org

The files served for the various versions on https://code.angularjs.org/
are retrieved by a Firebase function from a Firebase Storage bucket
(where they are deployed to from Travis CI). The files are stored
exactly as they are named on disk.

It turns out that some of the files have names with special characters
that get URI-encoded when sent to the Firebase function. For example,
`input[text].html` becomes `input%5Btext%5D.html`. As a result, the
actual file cannot be retrieved from the Storage bucket (since the name
does not match) and `index.html` is returned instead. Apparently, this
never worked, but nobody noticed or reported it until recently.

An example of a failing URL is:
https://code.angularjs.org/1.7.9/docs/api/ng/input/input%5Btext%5D

(NOTE: https://docs.angularjs.org/ works correctly, since the files are
deployed to Firebase hosting directly and not to a Storage bucket.)

This commit fixes the problem by decoding the request path before trying
to retrieve the corresponding file from the Storage bucket.

Closes #16943