Change defaults for fidelity bond settings.

[AdamISZ]

Closes #1247.
This consists of a change to the default value of
bondless_makers_allowance (was removed), an inclusion of the bond value exponent into
the config that the user can alter, and a change to the default of that
value from 2 (implicit by quadratic) to 1.3.
Also updates the fidelity bond documentation to account for those
changes.

[AdamISZ]

Comment from @chris-belcher in #1247 (comment) :

Right now all the displays in the software use BTC² as a unit, we should move to something like FBU (short for Fidelity Bond Unit) instead, then we define FBU = BTC^1.3 or something. Previously people would read 100 BTC² and after the update they'd read something like 75 FBU (seventy-five fidelity bond units)

So units would have to be addressed in this PR

[AdamISZ]

Added a commit (which I will squash) to change the units displayed (BTC^n) as a function of the exponent the user has in their config.

Important note: while the fidelity bonds table and the orderbook table should be correct, the sybil resistance calculations are AFAIU now wrong, I am hoping that @chris-belcher can update that, or if not, we can temporarily disable the page until it's correct.

[AdamISZ]

Reverted the change to bondless_maker_allowance back to 0.125 but still contains the exponent change (in config, and to 1.3 by default).

This is based on us having very strong consensus on the latter but not the former.

Squashed to one commit in 044bef6 , will merge very shortly unless there are objections.