Fix permissions checked for "Add" button in epics tree

When vising an epic the epic tree won't show the Add button when the user has sufficient permissions to read the epic.

To add an issue the user needs to be able to read the epic and admin the issue but to display the button we are checking if the user can admin the epic and thus not showing for users with access.

Note that we check different permissions for adding child epics, so we could hide the epics part of the menu if those permissions are not met.

Screenshots

Guest user	Reporter user

Expected behavior

• If the user can read the epic they can see the Add button that expands the menu and display the Issue options
• If the user can admin the epic and subepics feature is available display Epic options in the menu