Support an optional element as main picture of a corpus

• add a nullable FK from a project toward an element
• Allow setting that FK from UpdateCorpus

added 1 deleted label

mentioned in issue frontend#615 (closed)

assigned to @erwanrouchet

mentioned in merge request !1299 (merged)

closed

mentioned in commit 32cadfea

mentioned in issue #1328 (closed)

mentioned in issue frontend#1038 (closed)