Modern Events Calendar - Ticket and Invoice 3.4.0

- Fixed: Invoice Number option
- Fixed: Edit invoice details
- Fixed: Emails
- Fixed: Check-in user

- Added: Two capabilities mec_invoice_cap , mec_invoice_cap_others to manage invoices for users.

- Added: Invoice number in export bookings
- Added: Filter invoices with checkin status
- Added: Deactivate option for "PDF Download" button in the emails
- Improved: Download invoice button style
- Fixed: Sync booking and invoice status
- Fixed: Change users email in backend and send invoice to correct them