Remove concept of main accounts

[sunkup]

Purpose

Right now a LocalAddressBook still holds a reference to it's main account, but this is obsolete. We already have a relation between an address book account and the main account it belongs to via the corresponding database collection:

"main" account ↔ DB collection ↔ address book ↔ address book account

In the future we want to rely on system accounts as little as possible. Therefore this PR removes the direct relation between an address book account to its "main" account where possible.

Short description

• Acquire account settings via address book account - Adddress book settings are still stored in the settings of the main account.
• Create address book account name only from collection info - Although it might be more userfriendly, we don't need the name of the main account here and instead of a hash we can use the collection ID for uniqueness.
• Remove the "mainAccount" property from LocalAddressBook
• Stop storing main account name and type in address book account settings
• Don't allow address book accounts to get settings, must be caldav account - which stores all the settings anyways

Note: We can probably not use getAccountsByTypeAndFeatures to query for an account belonging to a specific db collection. That's because I think we can not set those features ourselves - that is to add the collection ID. Also addAccountExplicitly which we want to user over the normal addAccount, does not seem to have the option to "require account features".

Checklist

• The PR has a proper title, description and label.
• I have self-reviewed the PR.
• I have added documentation to complex functions and functions that can be used by other modules.
• I have added reasonable tests or consciously decided to not add tests.

[rfc2822]

Very nice! We can do it even without bitfireAT/davx5#603.

A few thoughts. Haven't tried in detail but I think it should work.

We probably should also have a talk about this, but maybe you can already have a look.

[rfc2822]

Please have a look at the changes and requested changes.

[sunkup]

It's a lot. But I think it's actually coming along rather nicely now 👍

[rfc2822]

I hope it should be fine now. Can you do a final check? Then I can also have a last look and merge.

[sunkup]

I hope it should be fine now. Can you do a final check? Then I can also have a last look and merge.

I think we're good. I played around with a bit more as well and it seems to hold up 👍