BoardApi add support for EpicBoards

[jmini]

Follow up of #1221

Add support for Epic Boards: https://docs.gitlab.com/ee/api/group_epic_boards.html

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Rename the methods to be more clear and consistent:

• Project Issue board: ProjectIssueBoard in the methods instead of just Board (getProjectIssueBoard(..), updateProjectIssueBoard(..)...)
  • older methods are preserved and deprecated
• Group Issue board: GroupIssueBoard instead of GroupBoard (getGroupIssueBoard(..), updateGroupIssueBoard(..)...)
  • those methods were just added with BoardApi fixes and group support #1221 and are just renamed
• Group Epic board: GroupEpicBoard added with this pull request

[jmini]

Many of the GroupEpicBoard method are not implemented