I2P e-mail probably shouldn't include finch fields

[RByers]

When crafting an I2P e-mail, fields like "Finch feature name" and "Non-finch justification" are included, even though those aren't expected to be filled out until the dev trial stage.

Perhaps we should just omit those fields from the I2P e-mail? They don't have value until I2E / I2S stage anyway.

Flag name is also entered and may not be required yet. Perhaps we should move that one back from dev trial to prototype stage? IMHO the first CL should be adding a flag, so it's not uneasonable to have chosen a flag name by the time of an I2P thread (which may have an unlanded initial CL).

[SamFort-MSFT]

This is also discussed in #3677.

Makes sense to me that feature flag names (base:Feature strings used to enable/disable a feature via --enable-features) would be part of a I2P since the code changes should be gated behind it. But about://flags and experimentation-specific names (finch) could come later during I2E.