Add link protocol field customization

[jacekbogdanski]

What is the purpose of this pull request?

New feature

Does your PR contain necessary tests?

All patches which change the editor code must include tests. You can always read more
on PR testing,
how to set the testing environment and
how to create tests
in the official CKEditor documentation.

This PR contains

• Unit tests
• Manual tests

What is the proposed changelog entry for this pull request?

[#2227](https://github.com/ckeditor/ckeditor-dev/issues/2227): Added [`config.link_defaultProcotol`](https://ckeditor.com/docs/ckeditor4/latest/api/CKEDITOR_config.html#cfg-link_defaultProtocol) configuration option that allows setting default URL protocol for [Link](https://ckeditor.com/cke4/addon/link) plugin dialog.

What changes did you make?

Closes #2227

[f1ames]

Unit tests

• Chrome (CI) 🚨 - see CI
• Firefox (CI) 🚨 - see CI
• Safari ✅
• IE8 ✅
• IE11 ✅
• Edge ✅

http://localhost:1030/#tests/is:unit,path:/tests/plugins/link,path:/tests/plugins/autolink,path:/tests/plugins/tableselection/integrations/link

Manual tests

1. There is an issue with <other> protocol - after selecting it, and reopening dialog default one is used:

2. Setting non-existent value or <other> results in empty Protocol value:

I'm for adding list of available values to link_defaultProtocol property docs, mentioning also that it can't handle custom values. I was thinking that this list should also include <other>, however it seems it is a replacement for empty/no-protocol and it has no value so might problematic to set:

Let's skip it.

Code

See code comments.

[f1ames]

Looks good 👍 And I see handling <other> was easier than I thought 👏

---

Unit tests

Chrome (CI) ✅
Firefox (CI) ✅
Safari ✅
IE8 ✅
IE11 ✅
Edge ✅

Manual tests

👍