WIP: allow multiple domains for IMAP restriction

[violoncelloCH]

thanks to @bjoern86 (nextcloud/apps#33):

Fixes #21

Changes proposed in this pull request:

• allow a comma seperated list of domains as input and if multiple domains are present, allow all of them
• also only stripe the domain if there is just one domain specified to avoid conflicts

however as mentioned in https://github.com/nextcloud/apps/pull/33#issuecomment-494903843 I'm not sure if this is the right approach to go... if we go for this approach I would change a few things:

• multiple domains should be taken as an array
• make the striping of the domain optional (at least if there is only one domain specified) but maybe also allow it as opt in for multiple domains (admin needs to avoid/prevent conflicts in this case)

[DJCrashdummy]

i'm not a dev, but i'm not sure if it is a good idea to deal with multiple domains within one backend entry... IMHO it just makes the code and the config more complex, although you can already achieve the same with simply adding more backend entries (one per domain) in the user_backends-array.

no idea if this approach improves the performance, even not at how much domains resp. backend entries it starts to suffer... but for now i can not think of an other reason why it may be necessary to implement this nevertheless. :thinking:

but as already said: i'm not a developer and these are just my 2 cents!

[violoncelloCH]

yes, I agree, let's close this!