support {header()} in Custom Background URLs

[jidanni]

Please see
https://help.openstreetmap.org/questions/55267/strava-heatmap-does-not-appear-anymore?page=1&focusedAnswerId=55381#55381

[jidanni]

Also at the bottom of https://josm.openstreetmap.de/wiki/Maps

Normally JOSM ignores any elements that it does not recognize. However, it will skip the entire imagery entry, when one of the unsupported tags has the attribute mandatory='true'.

So iD, instead of sending such {header()} etc. unknown elements down the pipe, still hanging on to the back of the URL, should at least strip them...

[bhousel]

Unfortunately that's not really possible to adjust the referrer header from within a web application. The browser sets it for us and we can't change it. Sorry!

This also came up for discussion on #3420

[jidanni]

OK, but it "behooves" you to throw an error:

Dear User: you tried to use a directive(s) [norflesburg, header] that we
 did not understand or cannot support, within your URL template.

Rather than sending your directive, still embedded in the URL
uninterpreted, to the server, which would be ** NEVER what your intended
** and indeed ** possibly quite embarrassing ** and just plain looks silly too,
please instead clean up your URL template, and reesubmit it.

[bhousel]

OK, but it "behooves" you to throw an error:

Isn't raising errors about custom imagery covered in one of the other half dozen issues you just opened?

[jidanni]

I don't see any for "catch unsupported URL template directives (that
should never be sent raw)'

Wait, I found it........