Option to allow ignoring invalid From header

[lieser]

Thunderbird itself can still work with messages that contain an invalid From header and instead silently fixes them.
Currently the addon shows an error in this case, but a user may want to ignore such errors.

The parsed from address is used for:

• Checking sign rules
  • E.g. if an email must be signed
• Checking alignment of the from address and the SDID/AUID
• Sorting of results

If there is a DKIM signature the error could be made into a warning.
For unsigned mails it would have to be ignored completely.