Prevent URLs from being entered containing stray spaces

[rhigman]

It is possible to enter a value https://creativecommons.org/ licenses/by-nc/4.0/ in the work Licence field (note space between .org/ and licences), even though both the app and the database have constraints which should prevent invalid URL values being entered.

This is not caused by #376 as the behaviour is the same when both adding and editing works. Similar issues exist for the Landing Page, Cover URL, and publication location Landing Page and Full Text URL fields. The bug is presumably in our generic implementation of URL constraints.

[rhigman]

This is slightly complicated as unencoded URLs can validly contain spaces (especially within parameters, e.g. https://twitter.com/search?q="open access"), but they are more safely represented encoded (the example would become https://twitter.com/search?q=%22open%20access%22). The app constraint simply uses the standard HTML <input type="url"> attribute, which accepts spaces for this reason. It is unclear whether it would be appropriate for us to update the database constraint to disallow spaces.