Configurable OpenSkill parameters (tau, beta...)

[StanczakDominik]

When I was changing tau for the S1 ladder reset, I basically simply hard-coded that in my fork of OpenSkill.ex as quick fix, see https://github.com/StanczakDominik/openskill.ex/blob/163a72f7423b8aa964909ea6aa3e9943739d87a6/lib/openskill/environment.ex#L7 . Ideally these should be passed in as options, and thankfully we actually already have that capability - https://github.com/StanczakDominik/openskill.ex/blob/163a72f7423b8aa964909ea6aa3e9943739d87a6/lib/openskill.ex#L29-L38

This is a simple fix with some interesting aspects so I'll mark it as a good first issue for a new contributor:

• ensure these can be set Teiserver-side, say in http://localhost:4000/teiserver/admin/site#Openskill (that #Openskill subsection doesn't yet exist) and have okay descriptions
• ensure they're passed to Openskill.ex wherever that's called (3 entries afaik!) and that the results change appropriately
• ensure that any current tests of that functionality we have teiserver-side still use the current values and are resilient to changing those config values

[L-e-x-o-n]

Make configurable:

• tau
• prevent_sigma_increase
• z
• mu
• epsilon