Compare lower case email when checking invitees

When comparing email to check if it is present in the invitees list, we do not normalize the value.
This leads to inconsistancies:
- the user will see two options when searching for alice@exampe.com if the stored email is Alice@example.com
- the user will see one option when searching for alice@exampe.com if the stored email is alice@example.com

This PR transform the values to lower cases to consolidate the number of options offered to the user.

Signed-off-by: Louis Chemineau <louis@chmn.me>