Fix EntityType creation and deletion in the Django admin

Closes #1465 (closed)

While testing the deletion in the admin, I found that I could not create an EntityType in the admin either since the corpus was not editable, so I fixed that as well.

changed milestone to %Arkindex 1.4.1

requested review from @babadie

assigned to @erwanrouchet

added 5 commits

• ff564812...454da644 - 2 commits from branch master
• a52b8f00 - Add corpus in EntityType list
• c88ed0c9 - Make the corpus a create-only field
• 0653d1d0 - Prevent deleting an EntityType

Compare with previous version

merged