Adding Email Address Validation When Inviting Squad Moderators

[staciajohanna]

Currently, any text that's inputted in the form of squad moderators invitation is accepted as email addresses. Preventing random texts to be accepted will be helpful. Should give meaningful error message if the user tries to put themselves as the moderators, or if the email address is not in a valid format.

[staciajohanna]

Will expand this to other forms that use email (e.g. whitelist/blacklist, etc.)