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
API listing endpoint filter can be found via &filter[name]=my+book query
parameters. There are a range of operators that can be used such as
&filter[id:gte]=4