If a button is not of type button, pressing enter inside an `<input>` inside a `<form>` without the action attribute will trigger the first available `<button>` as most browsers default the type of an unspecified button to submit.
This commit also prevents d-editor-modal to be filled when it's hidden.
Note this commit leaves out the biggest occurrence of the editor
which is the post/topic composer.
To avoid major breakage, this replaces it everywhere else it was
used:
* User preferences (About Me)
* Admin Customizations > Text Content
* Category Templates
* Editing Queued Posts
A future commit will replace the main composer with this editor
and will remove the unused pagedown code.