Mail match is currently case sentive

Problem/Motivation

When mail address is currently not exactly case sensitive at least "assign_auto" will not work.

Steps to reproduce

Just try a mail address which is a little bit different between user mail entry and grant invitation with uppercase chars.

Proposed resolution

Replacing "user_load_by_mail" will be a good start for 'assign_auto'. And check other strategies.

Remaining tasks

- Replace "user_load_by_mail" with a case insensitive database call
- Check other strategies about mail address search.

User interface changes

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API changes

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Data model changes