[common] Insist on organiser name/slug during init

Closes #451

doc/changelog.rst

@@ -3,6 +3,7 @@
 Release Notes
 =============
 
+- :bug:`451` Don't crash during ``pretalx init`` if no conference organiser slug is given.
 - :release:`0.7.1 <2018-06-19>`
 - :bug:`-` The new read-only links for submissions received the same secret token when migrating the database. No data was leaked as this made using the read-only links impossible. When upgrading to the next release, all read-only link addresses will be reset.
 - :bug:`-` A one-character-oversight led to issues with the new navigation search with certain user permissions.

src/pretalx/common/management/commands/init.py

@@ -11,6 +11,13 @@
 from pretalx.person.models import User
 
 
+def get_nonempty(prompt):
+    result = input(prompt).strip()
+    while not result:
+        result = input(_('This value is required, please enter some value to proceed: '))
+    return result
+
+
 class Command(BaseCommand):
     help = 'Initializes your pretalx instance. Only to be used once.'
 
@@ -26,8 +33,8 @@ def handle(self, *args, **options):
 
         self.stdout.write(_('''\nLet\'s also create a first organiser: This will allow you to invite further people and create events.'''))
 
-        organiser_name = input(_('\nName (e.g. "The Conference Organiser"): '))
-        organiser_slug = input(_('Slug (e.g. "conforg"): '))
+        organiser_name = get_nonempty(_('\nName (e.g. "The Conference Organiser"): '))
+        organiser_slug = get_nonempty(_('Slug (e.g. "conforg", used in urls): '))
 
         organiser, team = create_organiser_with_user(name=organiser_name, slug=organiser_slug, user=user)