Previously we patched the jqueryui-autocomplete library in order to
customize how the Tab and Enter keys behaved. Specifically, we wanted to
prevent the Tab key from moving the focus out of the tag input box, and
we wanted to prevent the Enter key from submitting the page when editing tags.
These things can achieved without patching the library by using
`event.preventDefault` and `event.stopImmediatePropagation` to prevent
other event handlers from running after these keys trigger the
`autocompleteselect` event.
Fix bug in the double submit prevention from #2789. Rather than
unbinding the enter key on submit, have the enter key click the submit
button instead. This avoids form submission when the submit button is disabled.
On the uploads page, trigger `Fetch source data` automatically when the
user pastes an URL into the source field. Silently ignore invalid URLs
so that we don't trigger spurious errors when the user types in a source
manually.
On the uploads page, don't call /artist/finder.json to find the artist
on page load. Instead have `Fetch source data` fill in the artist using
the `artists` data returned by the `/source.json` call.
* In the `Fetch source data` box, list the Danbooru artist tag(s) that
were found beside the artist name.
* Unify `Fetch source data` code that was duplicated between the
uploads page and the post show page.
* Use .on() instead of .bind() because bind is deprecated in jquery 3.0.
* Ensure enable-js-navigation is always respected.
* Namespace keybindings so they may be disabled by userscripts with
e.g. $(document).off("keydown.danbooru") or $(document).off("next_page").
