[UX] "Cancel" links and "Save and add another" buttons when adding vocabulary terms, menu items, and user accounts

As pointed out, a couple of years after this issue was originally created, the submit buttons for adding vocabulary terms and user accounts now automatically redirect you to the add form, but this functionality is missing from the menu item creation form. Also the add/edit menu link form does not have any "cancel" link, while the other two forms do (terms and user accounts that is).

Another thing is that there is some inconsistency in the labels of these buttons. For menu items and terms it says "save", while for user accounts it says "create new account" when adding a new account, or "save" when editing an existing one.

When testing and you simply want to add a single item, this "add another" feature can become annoying, but the existence of a "cancel" link in the subsequent form mitigates that. Ideally, we'd have separate "add another" buttons, with the simple "save" being the primary one (to match the behavior of the rest of the forms in the UI). In general, submit buttons should not behave in unexpected ways.

PS: I have also found that there are inconsistencies in the page titles and the breadcrumbs for these forms too, but I have split this to a separate issue: #3591

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Original issue summary

It's really annoying how adding a term takes you to the term list page and adding a menu link to the links list page - especially during the first stage of creating a website where you need to create the initial menu structure for example.

If we added "Save and add another" buttons, we'd save the user an extra click and page load. UX++

PS: We could consider adding such "Save and add" buttons in node creation forms too if people think that''d be useful (Add Another/Content Type: Extras/Submit Again).

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PR by @klonos: ...