Add insertUniqueEntity method

[cdepillabout]

This PR adds a insertUniqueEntity helper method.

This is a combination of insertEntity and insertUnique:

insertEntity :: record -> ReaderT backend m (Entity record)

insertUnique :: record -> ReaderT backend m (Maybe (Key record))

insertUniqueEntity :: record -> ReaderT backend m (Maybe (Entity record))

[psibi]

Can you add @since haddock markup for the introduced function add add corresponding changelog ?

[cdepillabout]

@psibi Thanks for the review. The @since annotation and changelog entry have been added in commit de5c7e1.

[psibi]

Thanks for the PR!