Set certain default settings per token

[missytake]

In many cases, we create a token for a certain group or organization, where we already know in which way they want to use Delta Chat - so it can make sense to change some of the default settings of the Delta Chat app, directly upon configuration.

Example:

• E-Corp is thinking about introducing Delta Chat as an internal messenger. They want to do an inhouse test first to collect employee feedback. Because E-Corp's facility has email bots which send out notifications for "smart-factory" functionality or whatever, all new users are supposed to also receive non-chat emails, not only chats. So we want to set this setting already upon configuration, so they don't have to change that setting manually for all users.

Proposed changes:

• add-token command gets a new parameter for setting preferences, similar to ssh -o StrictHostKeyChecking=no
• the same for mod-token
• check whether the supplied key/value pair is a correct config option, error if otherwise
• store the changed default config options as key/value pairs in the mailadm db
• put the key/value pairs into the response json when you scan an account creation QR code
• document changed json format

Changes needed in https://github.com/deltachat/deltachat-core-rust

• parse config options from json response during account configuration
• documentation for the different Delta Chat config options would be nice, too. Then we can link to it

[Simon-Laux]

also document the thing at https://github.com/deltachat/interface/blob/master/uri-schemes.md#the-http-endpoint-needs-the-following-api