Use hard line breaks in all default email texts

src/pretix/control/templates/pretixcontrol/email/email_setup.txt

@@ -8,7 +8,7 @@ If that was you, please enter the following confirmation code:
 
 If this was not requested by you, you can safely ignore this email.
 
-Best regards,
+Best regards,  
 
 Your {{ instance }} team
 {% endblocktrans %}

src/pretix/control/templates/pretixcontrol/email/forgot.txt

@@ -4,6 +4,6 @@ you requested a new password. Please go to the following page to reset your pass
 
 {{ url }}
 
-Best regards,
+Best regards,  
 Your pretix team
 {% endblocktrans %}

src/pretix/control/templates/pretixcontrol/email/invitation.txt

@@ -11,7 +11,7 @@ If you want to join that team, just click on the following link:
 
 If you do not want to join, you can safely ignore or delete this email.
 
-Best regards,
+Best regards,  
 
 Your pretix team
 {% endblocktrans %}

src/pretix/control/templates/pretixcontrol/email/security_notice.txt

@@ -11,6 +11,6 @@ You can review and change your account settings here:
 
 {{ url }}
 
-Best regards,
+Best regards,  
 Your pretix team
 {% endblocktrans %}