Various improvements

[denysvitali]

This is a huge PR (basically syncing my changes with yours), so let me know if you'd rather have me split it to make things easier.

My main branch features the following:

• Fix #66: It is now possible to use one of the User CAs to securely connect to the backend when a CA is not trusted by Flutter (but it's trusted by the system)
• Workaround for #67: add support for X-Client-Token, a special header that will be added to all the requests. To set this parameter, tap quickly 5 times the Vikunja logo in the login page to show the field
• Improve Landing Page view by adding Material Chips to show the project associated with the to-do item
• Strike-through the text of completed To-Do items
• Show a description / due date for the To-Do item
• Build unsigned builds via the CI (for forks that do not have access to your project's secrets)
• Update all the dependencies
• ... other things that I don't remember :D