Proto: no more rolls

Given that rolls only have one role, to set the minimal stake for participating in consensus and in voting, we make that more clear by renaming the constant tokens_per_roll to minimal_stake.

(There are many commits, but some could be squashed.)

Manually testing the MR

Checklist

• [n/a] Document the interface of any function added or modified (see the coding guidelines)
• [n/a] Document any change to the user interface, including configuration parameters (see node configuration)
• [n/a] Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR