Added test to cover.
Did attempt a 64k limit, but values over 2k significantly increase
chance of other issues since this URL may be used in redirect headers.
Would rather catch issues in-app.
For #4044
Replaces the old suggestion of setting JS head 'window.uploadLimit'
variable. This new env option will be used by back-end validation and
front-end libs/logic too.
Limits already likely exist within prod environments at a PHP and
webserver level but this allows an app-level limit and centralises the
option on the BookStack side into the .env
Closes#3033