Unify URL validation

URL validation takes place at several points in our system, namely in every Input element, which needs it.
This should be abstracted, so that error can be fixed in one place instead of a shotgun surgery.
Elements which currently do this on their own (which come to my mind) are:
`TextareaHTML` in `validatA()` and `validateFigure()`
`InputURL`
`TextareaLineURLArray` (inherits from `InputURL`)
