New UX insert link dialog - Insert anchors about silverstripe-admin

[tractorcow]

@clarkepaul question about the designs; I have spoken with @flamerohr and he suggested to look into a new approach.

Instead of a dropdown + textfield approach, could we use a single comobox (typeable dropdown) as a means of selecting an existing / creating a new value.

Change:

To:

A demo of how the combobox could work can be tested at https://jedwatson.github.io/react-select/ (bottom of page, select single select).

[flamerohr]

I believe the reason I had suggested it was too hard was because the campaigns would be mixing types of data - IDs (number) with Name of new campaign (string)
the anchor work is just string for new or existing, so it's much easier to deal with

[tractorcow]

Won't be able to get this done in time for beta3. Moved to next milestone.

WIP at https://github.com/open-sausages/silverstripe-cms/tree/pulls/4.0/anchor-modal

[clarkepaul]

Yeah that would be good if it was possible. I thought we didn't have that UI possibility though as I already asked for this pattern to be introduced for adding items to campaigns (select or add new) but I was told it was too hard to do, I think @flamerohr said it was too hard to do.

[clarkepaul]

Thought it might be good to add this related issue as we were going to use the same approach as mentioned above but either we didn't have time to implement or wasn't quite appropriate at the time. @flamerohr has some tips for implementation also #161 (comment)

If we do use this functionality then we could retrospectively update the "add to campaign" to follow the same pattern?

[tractorcow]

Feedback from review is implemented.