Undo stolen issue from epic

Description

After adding issue(s) to an epic, if any of those issues were transferred from a previous epic, provide some messaging and action that allows the user to revert that move.

Proposal

Check all design variations and specs (for implementation notes, spacing, sizes, colors and text copying)

Warning	Undoing	Success	Error

• 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
• If there were issues moved from another epic:
  • If it was only one issue:
    • Show the warning message Issue <ISSUEREF> was moved from epic <EPICREF> to this one. [Undo]
    • The issue and epic references link to their respective objects and show a tooltip with their title
  • If it was more than one issue:
    • Show the warning message 2 issues were moved from other epics to this one. [Undo]
  • Pressing “Undo” moves the issues back to their previous epics. While in progress, replace the “Undo” link with a spinner.
• If the undo process succeeds, show the success message You moved that issue back to its previous epic or You moved those issues back to their previous epics, depending on the number of issues.
• If the undo process fails, show the error message Oops, could not undo due to server error. [Retry]
  • Pressing “Retry” repeats the undo process. While in progress, replace the “Retry” link with a spinner.
• For this issue, all messages stay on screen until the user refreshes the page, presses “Cancel”, or adds new issues to the list
• API: When adding an issue to an epic and consequently stealing away from another epic, respond to the request saying that the issue was indeed stolen and from what epic it was stolen