Add support for updating items via email

[hershi]

Today, once a work item has been created, the only thing that is updated when a new email arrives is the comments section (with the latest message).
This item is for supporting updates to item fields in follow up emails via mnemonics and explicit overrides - if the latest message in the thread contains any mnemonics or explicit overrides, those settings should be reflected in the item.
This would enable support for "directives" via mnemonics - i.e. if the admin wants to support a "Resolve" directive that would resolve the issue, they can define a mnemonic for "Resolve" that sets the state to "Resolved".

This support should include a configuration setting to decide whether that feature is enabled or not.

[hershi]

This is now supported. To activate it, the following config value needs to be set under the WorkItemSettings config section:
true

Once this is enabled, explicit overrides and mnemonics in reply messages are considered and relevant fields are updates.