Publishing flow: the post URL field needs to be properly labelled

[afercia]

In the publishing flow, the post URL is unlabelled. It misses a <label> element to give the field a name.

Worth reminding all input fields need to be properly labelled to communicate users what they're about.

See related #525, #557, #562, #694, #695, #906, #979, #1409, #1659, #1664, #1854, #1855, #4158, #5823, #6001, #6033, #6199, #6260, #6918, #8104.

Worth reminding there's a pending proposal to introduce automated tests to check for labels, currently filed under the "Ideas" project, see #6098