Factoring out the common parts of DotNest.Frontend to a new module

[humanoidcreature]

E.g. Lombiq.Hosting.Multitenancy.Frontend. All because of after this it can be a part of the Hosting Suite. Then add to this in DotNest.Frontend what is DotNest specific.

Now it only includes the tenant management parts.

For this data migration is required, which can be done in a obvious way like turning on the new module, delete its tables, and after that we rename the old DotNest.Frontend's tables to the new name. Along with this the UpdateFrom methods can be erased, thus this will be a new module.