Update threat model for Linear Leios (#486)

* Update system description in threat model

This should characterize the latest protocol design (linear leios)
sufficiently for the threat model.

* Remove headings and update T1 likelihood

* Update and rename EB/Transaction withholding

* Move unnumbered threats to the bottom

* Detail front running and realize we cannot mitigate this

* Over-parameterization not possible anymore with linear leios

* Add threat numbers to all threats

* Add mitigation of no conflicting transactions

* Drop the risk assessment matrix for now

* Fix typos

Author: ch1bo