Validation error when adding issue to epic or related issues

• Update both epics and related issues.
• Update the placeholder to be simply Issue URL(s) (example)
• Whenever there is any content in the textbox, remove the placeholder text.
• When you click “Add”:
  • Disable the “Cancel” button during processing
  • Hide the “Add” button label and show a progress spinner centered on the button, per the guidelines in gitlab-design#82
• When adding multiple issue(s), persist the valid issues. (This is existing functionality.)
• For the invalid issues/URLs, leave them in the textbox, and show an inline error message.
• In the error state, the textbox should adopt the common error style border.
• The existing banner error message should be removed.
• The error message should be different for epics and related issues — see below
  • For this iteration, we are using one generic error message for each scenario.

Epic error message	Related issues error message
Use properly formatted URLs of issues that you can edit and exist in projects of this group	Use properly formatted URLs of issues that you can edit