[WIP] Allow to configure a custom event domain

[rixx]

This PR closes/references issue #171. It is still work in progress – the custom Url class has been added, now we need a settings field, and a middleware handling security concerns since we need to set ALLOWED_HOSTS = ['*'].

We'll need pretty much everything the pretix multidomain app provides, so we should talk to @raphaelm if he things separating parts of it from pretix/core makes sense, or if we'll just carry code around as per usual.

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• My change is listed in the CHANGELOG.rst if appropriate.

[codecov]

Codecov Report

Merging #252 into master will decrease coverage by 0.05%.
The diff coverage is 89.4%.

@@            Coverage Diff             @@
##           master     #252      +/-   ##
==========================================
- Coverage   87.82%   87.77%   -0.06%     
==========================================
  Files          83       84       +1     
  Lines        4092     4204     +112     
  Branches      424      445      +21     
==========================================
+ Hits         3594     3690      +96     
- Misses        360      368       +8     
- Partials      138      146       +8

Impacted Files	Coverage Δ
src/pretalx/common/middleware/event.py	79.06% <ø> (ø)
src/pretalx/cfp/forms/submissions.py	85% <ø> (ø)	⬆️
src/pretalx/cfp/views/auth.py	84.05% <ø> (ø)	⬆️
src/pretalx/event/models/event.py	96.81% <100%> (ø)	⬆️
src/pretalx/submission/models/question.py	97.46% <100%> (ø)	⬆️
src/pretalx/agenda/views/feed.py	100% <100%> (ø)	⬆️
src/pretalx/submission/models/submission.py	93.22% <100%> (ø)	⬆️
src/pretalx/mail/models.py	94.36% <100%> (ø)	⬆️
src/pretalx/submission/models/review.py	100% <100%> (ø)	⬆️
src/pretalx/submission/models/cfp.py	88.88% <100%> (ø)	⬆️
... and 10 more