[PP-1] Convert embed dialog to a single step form

Problem/Motivation

Currently the embed form is a two-step form which poses various problems:

• Issue with Back and Next
• Not a very intuitive UX
• Disconnected with 'Insert Image' dialog
• Lots of hacky code to handle two steps of form

Proposed resolution

The idea is to convert it to single insert dialog similar to the one used to insert images. The workflow will be something like this:

• The entity ID/UUID textbox at the the top. when user 'inserts' an entity, the textbox changes to a link to the entity like Core's 'managed file' element.
• All other options, like display plugins etc. which were initially empty are now populated with the options for the selected entity using AJAX framework.
• User can 'Remove' the entity and all these options again go back to empty.

This approach will make the UI/UX better and also bring the embed dialog more in line with the Core's Image dialog. This will also get rid of the ugly errors that are brought by Back/Next buttons.

Remaining tasks

• Agree on a solution.
• Fix it.
• Test it.