* Upgrade Django to 3.0 and other dependencies to recent versions
* Fix otp version contsraint
* Remove six dependency
* Resolve some warnings
* Fix failing tests
* Update django-countries
* Resolve all RemovedInDjango31Warnings in test suite
* Run isort
* Fix import
* Update PostgreSQL version on travis
* TicketOutput-Providers: Make preview optional; download/attachable optional; optional specific target; update doc
* Spelling fixes in doc
* Changes after code-review
* Changes after code-review
* Commit missing template file
* Allow for redirects instead of files
* Return HTTPResponse with Content-Type text/uri-list on API
* Update API-doc
* Add viewable to spellinglist, fixing doc-test
* Added non-admission setting to event
`ticket_download_nonadm` now setting in storage. Still need logic for
order page/PDF generation.
Works on pretix/pretix#456.
* Download button considers `ticket_download_nonadm`
Modified Django tags to look at item admission attribute and
`ticket_download_nonadm` setting.
Works on pretix/pretix#456.
* Ticket output for non-admission disabled
PDFs/etc. will only be permitted/generated for items with the
`admission` attribute, or if the `ticket_download_nonadm` event setting
is true. Applies to single and whole-order ticket downloads.
Works on pretix/pretix#456.
* Fixed product exclusion in PDF output
Forgot PDF output was a plugin, now includes same check as base
`BaseTicketOutput.generate_order`.
Works on pretix/pretix#456
