feat(invite) add email autocomplete in invite

[raphaelbadawi]

This is the follow up to this discussion in which the specs were discussed.

This adds an autocomplete for mere emails. Since emails are not Jitsi entity types, this also adds an alternative authentication mechanism for directories holding external entities (this authentication mechanism was also discussed in the issue above).

The documentation for this has been written too, here is the PR for this part => jitsi/handbook#503

Thanks!

[jitsi-jenkins]

Hi, thanks for your contribution!
If you haven't already done so, could you please make sure you sign our CLA (https://jitsi.org/icla for individuals and https://jitsi.org/ccla for corporations)? We would unfortunately be unable to merge your patch unless we have that piece :(.

[raphaelbadawi]

Hello,

Here is the CLA signed by my CEO.

8x8_Corporate_CLA_rev.20181129.docx.pdf

Dev list.pdf

Best regards,
Raphaël.